Radio access network node, radio terminal, and methods therefor

ABSTRACT

A master RAN node (1) associated with a master RAT (1) communicates with a secondary RAN node (2) associated with a secondary RAT and provides a radio terminal (3) with dual connectivity that uses the master RAT and the secondary RAT. In response to receiving, from the radio terminal (3) or a core network (4), terminal capability information indicating that the radio terminal (3) supports the split bearer, the master RAN node (1) uses a PDCP entity, which provides unified PDCP functionalities, for a master cell group split bearer for the radio terminal (3).

TECHNICAL FIELD

The present disclosure relates to a radio communication system and, inparticular, to communication in which a radio terminal simultaneouslyuses a plurality of cells of different Radio Access Technologies (RATs)operated by different radio stations (multi-connectivity operation).

BACKGROUND ART

The 3rd Generation Partnership Project (3GPP) has been conducting thestandardization for the fifth generation mobile communication system(5G) to make it a commercial reality in 2020 or later. 5G is expected tobe realized by continuous enhancement/evolution of LTE and LTE-Advancedand an innovative enhancement/evolution by an introduction of a new 5Gair interface (i.e., a new Radio Access Technology (RAT)). The new RATsupports, for example, frequency bands higher than the frequency bands(e.g., 6 GHz or lower) supported by LTE/LTE-Advanced and its continuousevolution. For example, the new RAT supports centimeter-wave bands (10GHz or higher) and millimeter-wave bands (30 GHz or higher).

In this specification, the fifth generation mobile communication systemis referred to as a 5G System or a Next Generation (NextGen) System (NGSystem). The new RAT for the 5G System is referred to as a New Radio(NR), a 5G RAT, or a NG RAT. A new Radio Access Network (RAN) for the 5GSystem is referred to as a 5G-RAN or a NextGen RAN (NG RAN). A new basestation in the 5G-RAN is referred to as a NR NodeB (NR NB) or a gNodeB(gNB). A new core network for the 5G System is referred to as a 5G CoreNetwork (5G-CN or 5GC) or a NextGen Core (NG Core). A radio terminal(i.e., User Equipment (UE)) capable of being connected to the 5G Systemis referred to as 5G UE or NextGen UE (NG UE), or simply referred to asUE. The official names of the RAT, UE, radio access network, corenetwork, network entities (nodes), protocol layers and the like for the5G System will be determined in the future as standardization workprogresses.

The term “LTE” used in this specification includes enhancement/evolutionof LTE and LTE-Advanced to provide interworking with the 5G System,unless otherwise specified. The enhancement/evolution of LTE andLTE-Advanced for the interworking with the 5G System is referred to asLTE-Advanced Pro, LTE+, or enhanced LTE (eLTE). Further, terms relatedto LTE networks and logical entities used in this specification, such as“Evolved Packet Core (EPC)”, “Mobility Management Entity (MME)”,“Serving Gateway (S-GW)”, and “Packet Data Network (PDN) Gateway(P-GW))”, include their enhancement/evolution to provide interworkingwith the 5G System, unless otherwise specified. Enhanced EPC, enhancedMME, enhanced S-GW, and enhanced P-GW are referred to, for example, asenhanced EPC (eEPC), enhanced MME (eMME), enhanced S-GW (eS-GW), andenhanced P-GW (eP-GW), respectively.

In LTE and LTE-Advanced, for achieving Quality of Service (QoS) andpacket routing, a bearer per QoS class and per PDN connection is used inboth a RAN (i.e., an Evolved Universal Terrestrial RAN (E-UTRAN)) and acore network (i.e., EPC). That is, in the Bearer-based QoS (orper-bearer QoS) concept, one or more Evolved Packet System (EPS) bearersare configured between a UE and a P-GW in an EPC, and a plurality ofService Data Flows (SDFs) having the same QoS class are transferredthrough one EPS bearer satisfying this QoS. An SDF is one or more packetflows that match an SDF template (i.e., packet filters) based on aPolicy and Charging Control (PCC) rule. In order to achieve packetrouting, each packet to be transferred through an EPS bearer containsinformation for identifying which bearer (i.e., General Packet RadioService (GPRS) Tunneling Protocol (GTP) tunnel) the packet is associatedwith.

In contrast, with regard to the 5G System, it is discussed that althoughradio bearers may be used in the NG-RAN, no bearers are used in the 5GCor in the interface between the 5GC and the NG-RAN (see Non-PatentLiterature 1). Specifically, PDU flows are defined instead of an EPSbearer, and one or more SDFs are mapped to one or more PDU flows. A PDUflow between a 5G UE and a user-plane terminating entity in an NG Core(i.e., an entity corresponding to a P-GW in the EPC) corresponds to anEPS bearer in the EPS Bearer-based QoS concept. The PDU flow correspondsto the finest granularity of the packet forwarding and treatment in the5G system. That is, the 5G System adopts the Flow-based QoS (or per-flowQoS) concept instead of the Bearer-based QoS concept. In the Flow-basedQoS concept, QoS is handled per PDU flow. In the QoS framework of the 5Gsystem, a PDU flow is identified by a PDU flow ID contained in a headerencapsulating a Service Data Unit of a tunnel of a NG3 interface. TheNG3 interface is a user plane interface between the 5GC and the gNB(i.e., NG-RAN). Association between a 5G UE and a data network isreferred to as a “PDU session”. The term “PDU session” corresponds tothe term “PDN connection” in LTE and LTE-Advanced. A plurality of PDUflows can be configured in one PDU session.

The PDU flow is also referred to as a “QoS flow”. The QoS flow is thefinest granularity in QoS treatment in the 5G system. User plane traffichaving the same NG3 marking value in a PDU session corresponds to a QoSflow. The NG3 marking corresponds to the above-described PDU flow ID,and it is also referred to as a QoS flow ID or a Flow IdentificationIndicator (FII).

FIG. 1 shows a basic architecture of the 5G system. A UE establishes oneor more Signalling Radio Bearers (SRBs) and one or more Data RadioBearers (DRBs) with a gNB. The 5GC and the gNB establish a control planeinterface and a user plane interface for the UE. The control planeinterface between the 5GC and the gNB (i.e., RAN) is referred to as anNG2 interface or an NG-c interface and is used for transfer ofNon-Access Stratum (NAS) information and for transfer of controlinformation (e.g., NG2 AP Information Element) between the 5GC and thegNB. The user plane interface between the 5GC and the gNB (i.e., RAN) isreferred to as an NG3 interface or an NG-u interface and is used fortransfer of packets of one or more PDU flows in a PDU session of the UE.

Note that the architecture shown in FIG. 1 is merely one of the 5Garchitecture options (or deployment scenarios). The architecture shownin FIG. 1 is referred to as “Standalone NR (in NextGen System)” or“Option 2”. The 3GPP further discusses several network architectures formulti-connectivity operations using the E-UTRA and NR radio accesstechnologies. The multi-connectivity operation using the E-UTRA and NRradio access technologies is referred to as Multi-RAT Dual Connectivity(MR-DC). The MR-DC is dual connectivity between E-UTRA and NR nodes.

In the MR-DC, one of the E-UTRA node (i.e., eNB) and the NR node (i.e.,gNB) operates as a Master node (MN) and the other one operates as aSecondary node (SN), and at least the MN is connected to the corenetwork. The MN provides one or more Master Cell Group (MCG) cells tothe UE, while the SN provides one or more Secondary Cell Group (SCG)cells to the UE. The MR-DC includes “MRDC with the EPC” and “MRDC withthe 5GC”.

The MRDC with the EPC includes E-UTRA-NR Dual Connectivity (EN-DC). Inthe EN-DC, the UE is connected to an eNB operating as the MN and a gNBoperating as the SN. Further, the eNB (i.e., Master eNB) is connected tothe EPC, while the gNB (i.e. Secondary gNB) is connected to the MastereNB through the X2 interface.

The MRDC with the 5GC includes NR-E-UTRA Dual Connectivity (NE-DC) andE-UTRA-NR Dual Connectivity (NG-EN-DC). In the NE-DC, the UE isconnected to a gNB operating as the MN and an eNB operating as the SN,the gNB (i.e., Master gNB) is connected to the 5GC, and the eNB (i.e.Secondary eNB) is connected to the Master gNB through the Xn interface.On the other hand, in the NG-EN-DC, the UE is connected to an eNBoperating as the MN and a gNB operating as the SN, the eNB (i.e., MastereNB) is connected to the 5GC, and the gNB (i.e. Secondary gNB) isconnected to the Master eNB through the Xn interface.

FIGS. 2, 3, and 4 show network configurations of the above-describedthree DC types, i.e., EN-DC, NE-DC, and NG-EN-DC, respectively. FIG. 5shows SRBs and DRBs supported by these three DC types. Not that, FIG. 5shows bearer types to be supported in 3GPP Release 15 that is currentlyunder discussion in the 3GPP. Accordingly, the bearer types supported bythe three DC types may be different from those shown in FIG. 5.

The MCG SRB is an SRB established between the UE and the MN. RadioResource Control Protocol Data Units (RRC PDUs) generated by the SN canbe transported to the UE via the MN and the MCG SRB. Alternatively, theUE is able to establish an SRB (SCG SRB) with the SN in order totransport RRC PDUs for the SN directly between the UE and the SN. TheMCG split SRB enables duplication of RRC PDUs generated by the MN.

The MCG bearer is a user plane bearer whose radio protocols are onlylocated in the MCG. The MCG split bearer is a user plane bearer whoseradio protocols are split at the MN and belong to both the MCG and theSCG. The SCG bearer is a user plane bearer whose radio protocols areonly located in the SCG. The SCG split bearer is a user plane bearerwhose radio protocols are split at the SN and belong to both the SCG andthe MCG.

Note that, the layer 2 functionality of the gNB (NR) is not the same asthe layer 2 functionality of the eNB (LTE). For example, the layer 2 ofthe gNB (NR) includes four sublayers, i.e., a Service Data AdaptationProtocol (SDAP) sublayer, a Packet Data Convergence Protocol (PDCP)sublayer, a Radio Link Control (RLC) sublayer, and a Medium AccessControl (MAC) sublayer. In the NR PDCP sublayer, the size of the PDCPSequence Number (SN) for DRBs is 12 bits or 18 bits, which is a subsetof the possible values for the size of LTE PDCP SN (i.e., 7 bits, 12bits, 15 bits, or 18 bits). However, when the eNB (LTE) is connected tothe 5GC, the layer 2 of the eNB (LTE) includes an SDAP sublayer.

The 3GPP is also discussing an introduction of a unified split bearer.The purpose of the introduction of the unified split bearer is to usethe same protocols, configurations, and procedures for both the MCG andSCG split bearers as much as possible, thereby simplifying thespecification and UE implementation. As specific means for thisintroduction, a common (single) PDCP layer has been proposed (seeNon-Patent Literature 1). The common PDCP layer supports both the MCGand SCG split bearers. For example, the common PDCP layer may be thesame as a PDCP layer (NR PDCP layer) used for the NR standaloneoperation.

Non-Patent Literature 1 proposes that it should be possible to switchbetween split and no-split bearers without re-establishing PDCP in caseswhere the PDCP termination point is not moved. Non-Patent Literature 1further proposes that it should be possible to configure the same PDCPversion as used for split bearers already when the UE operates in LTEonly, to allow switching to and from split bearers without PDCPre-establishment due to protocol change.

CITATION LIST Non Patent Literature

-   [Non-Patent Literature 1] 3GPP Tdoc R2-1704414, Ericsson, “On the    different bearer options”, 3GPP TSG-RAN WG2 Meeting #98, May 2017

SUMMARY OF INVENTION Technical Problem

As described above, Non-Patent Literature 1 proposes a common (single)PDCP layer that supports both the MCG and SCG split bearers. The commonPDCP layer may be referred to as a unified PDCP layer. However, it isnot clear how the common (or unified) PDCP layer should be implementedin a radio communication network (e.g., 3GPP network).

One of the objects to be attained by embodiments disclosed herein is toprovide an apparatus, a method, and a program that assist inimplementation of a common (or unified) PDCP layer in a radiocommunication network. It should be noted that this object is merely oneof the objects to be attained by the embodiments disclosed herein. Otherobjects or problems and novel features will be made apparent from thefollowing description and the accompanying drawings.

Solution to Problem

In a first aspect, a master RAN node associated with a master RATincludes a memory and at least one processor coupled to the memory. Theat least one processor is configured to communicate with a secondary RANnode associated with a secondary RAT and provide a radio terminal withdual connectivity that uses the master RAT and the secondary RAT. The atleast one processor is further configured to, in response to receiving,from the radio terminal or a core network, terminal capabilityinformation indicating that the radio terminal supports a split bearer,use a Packet Data Convergence Protocol (PDCP) entity, which providesunified PDCP functionalities, for a master cell group split bearer forthe radio terminal. The unified PDCP functionalities are used for boththe master cell group split bearer and a secondary cell group splitbearer. The master cell group split bearer is a user plane bearer whoseradio protocols are split at the master RAN node and belong to both amaster cell group provided by the master RAN node and a secondary cellgroup provided by the secondary RAN node. The secondary cell group splitbearer is a user plane bearer whose radio protocols are split at thesecondary RAN node and belong to both the secondary cell group and themaster cell group.

In a second aspect, a secondary RAN node configured to support asecondary RAT includes a memory and at least one processor coupled tothe memory. The at least one processor is configured to communicate witha master RAN node that supports a master RAT and provide a radioterminal with dual connectivity that uses the master RAT and thesecondary RAT. The at least one processor is further configured to, ifthe master RAN node receives, from the radio terminal or a core network,terminal capability information indicating that the radio terminalsupports a split bearer, use a Packet Data Convergence Protocol (PDCP)entity, which provides unified PDCP functionalities, for a secondarycell group split bearer for the radio terminal.

In a third aspect, a radio terminal includes at least one wirelesstransceiver and at least one processor. The at least one wirelesstransceiver is configured to communicate with both a master radio accessnetwork (RAN) node associated with a master radio access technology(RAT) and a secondary RAN node associated with a secondary RAT. The atleast one processor is configured to perform, via the at least onewireless transceiver, dual connectivity that uses the master RAT and thesecondary RAT. The at least one processor is further configured to, ifthe radio terminal supports a split bearer, transmit, to the master RANnode, terminal capability information indicating that the radio terminalsupports a split bearer. Furthermore, the at least one processor isconfigured to, if the radio terminal supports a split bearer, use aPacket Data Convergence Protocol (PDCP) entity, which provides unifiedPDCP functionalities, for a master cell group split bearer for the radioterminal.

In a fourth aspect, a method for a master RAN node associated with amaster RAT includes:

(a) communicating with a secondary RAN node associated with a secondaryRAT and providing a radio terminal with dual connectivity that uses themaster RAT and the secondary RAT; and

(b) in response to receiving, from the radio terminal or a core network,terminal capability information indicating that the radio terminalsupports a split bearer, using a Packet Data Convergence Protocol (PDCP)entity for a master cell group split bearer for the radio terminal, thePDCP entity providing unified PDCP functionalities.

In a fifth aspect, a method for a secondary RAN node configured tosupport a secondary RAT includes:

(a) communicating with a master RAN node that supports a master RAT andproviding a radio terminal with dual connectivity that uses the masterRAT and the secondary RAT; and

(b) if the master RAN node receives, from the radio terminal or a corenetwork, terminal capability information indicating that the radioterminal supports a split bearer, using a Packet Data ConvergenceProtocol (PDCP) entity for a secondary cell group split bearer for theradio terminal, the PDCP entity providing unified PDCP functionalities.

In a sixth aspect, a method for a radio terminal includes:

(a) performing dual connectivity that uses a master radio accesstechnology (RAT) and a secondary RAT via a wireless transceiverconfigured to communicate with both a master radio access network (RAN)node associated with the master RAT and a secondary RAN node associatedwith the secondary RAT;

(b) if the radio terminal supports a split bearer, transmitting, to themaster RAN node, terminal capability information indicating that theradio terminal supports a split bearer; and

(c) if the radio terminal supports a split bearer, using a Packet DataConvergence Protocol (PDCP) entity for a master cell group split bearerfor the radio terminal, the PDCP entity providing unified PDCPfunctionalities.

In a seventh aspect, a master RAN node associated with a master RATincludes a memory and at least one processor coupled to the memory. Theat least one processor is configured to communicate with a secondary RANnode associated with a secondary RAT and provide a radio terminal withdual connectivity that uses the master RAT and the secondary RAT. The atleast one processor is further configured to, if the radio terminal doesnot support a split bearer, use, for a master cell group bearer for theradio terminal, a PDCP entity that provides first Packet DataConvergence Protocol (PDCP) functionalities corresponding to the masterRAT. Furthermore, the at least one processor is configured to, if theradio terminal supports the split bearer, use, for the master cell groupbearer for the radio terminal, a PDCP entity that provides unified PDCPfunctionalities, regardless of whether the dual connectivity is startedfor the radio terminal. The master cell group bearer is a user planebearer whose radio protocols are only located in the master cell group.

In an eighth aspect, a radio terminal includes at least one wirelesstransceiver and at least one processor. The at least one wirelesstransceiver is configured to communicate with both a master radio accessnetwork (RAN) node associated with a master radio access technology(RAT) and a secondary RAN node associated with a secondary RAT. The atleast one processor is configured to perform, via the at least onewireless transceiver, dual connectivity that uses the master RAT and thesecondary RAT. The at least one processor is further configured to, ifthe radio terminal does not support a split bearer, use, for a mastercell group bearer for the radio terminal, a PDCP entity that providesfirst Packet Data Convergence Protocol (PDCP) functionalitiescorresponding to the master RAT. Furthermore, the at least one processoris configured to use, if the radio terminal supports the split bearer,use, for the master cell group bearer for the radio terminal, a PDCPentity that provides unified PDCP functionalities, regardless of whetherthe dual connectivity is started for the radio terminal.

In a ninth aspect, a method for a master RAN node associated with amaster RAT includes:

(a) communicating with a secondary RAN node associated with a secondaryRAT and providing a radio terminal with dual connectivity that uses themaster RAT and the secondary RAT;

(b) if the radio terminal does not support a split bearer, using, for amaster cell group bearer for the radio terminal, a PDCP entity thatprovides first Packet Data Convergence Protocol (PDCP) functionalitiescorresponding to the master RAT; and

(c) if the radio terminal supports the split bearer, using, for themaster cell group bearer for the radio terminal, a PDCP entity thatprovides unified PDCP functionalities, regardless of whether the dualconnectivity is started for the radio terminal.

In a tenth aspect, a method for a radio terminal comprises:

(a) performing dual connectivity that uses a master radio accesstechnology (RAT) and a secondary RAT via a wireless transceiverconfigured to communicate with both a master radio access network (RAN)node associated with the master RAT and a secondary RAN node associatedwith the secondary RAT;

(b) if the radio terminal does not support a split bearer, using, for amaster cell group bearer for the radio terminal, a PDCP entity thatprovides first Packet Data Convergence Protocol (PDCP) functionalitiescorresponding to the master RAT; and

(c) if the radio terminal supports the split bearer, using, for themaster cell group bearer for the radio terminal, a PDCP entity thatprovides unified PDCP functionalities, regardless of whether the dualconnectivity is started for the radio terminal.

In an eleventh aspect, a program includes instructions (software codes)that, when loaded into a computer, cause the computer to perform themethod according to the above-described fourth, fifth, sixth, ninth, ortenth aspect.

Advantageous Effects of Invention

According to the above-deceived aspects, it is possible to provide anapparatus, a method, and a program that assist in implementation of acommon (or unified) PDCP layer in a radio communication network.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a basic architecture of a 5G System;

FIG. 2 is a diagram showing a network configuration of EN-DC;

FIG. 3 is a diagram showing a network configuration of NE-DC;

FIG. 4 is a diagram showing a network configuration of NG-EN-DC;

FIG. 5 is a table indicating bearer types supported by the three DCtypes currently under discussion in the 3GPP;

FIG. 6 is a diagram showing a configuration example of a radiocommunication network according to a plurality of embodiments;

FIG. 7 is a diagram showing radio protocol architecture for a splitbearer according to a plurality of embodiments;

FIG. 8 is a sequence diagram showing one example of a procedure forestablishing an MCG split bearer according to a first embodiment;

FIG. 9 is a sequence diagram showing one example of a procedure forestablishing an SCG split bearer according to the first embodiment;

FIG. 10 is a sequence diagram showing one example of an MR-DC startingprocedure involving establishment of an MCG split bearer according to asecond embodiment;

FIG. 11 is a sequence diagram showing one example of an MR-DC startingprocedure involving establishment of an SCG bearer or an SCG splitbearer according to the second embodiment;

FIG. 12 is a sequence diagram showing one example of a procedure forestablishing an RRC connection and a user plane bearer according to afourth embodiment;

FIG. 13 is a sequence diagram showing one example of a procedure forestablishing an RRC connection and a user plane bearer according to thefourth embodiment;

FIG. 14 is a block diagram showing a configuration example of a masternode according to a plurality of embodiments;

FIG. 15 is a block diagram showing a configuration example of a UEaccording to a plurality of embodiments; and

FIG. 16 is a block diagram showing a configuration example of a corenetwork node according to a plurality of embodiments.

DESCRIPTION OF EMBODIMENTS

Specific embodiments will be described hereinafter in detail withreference to the drawings. The same or corresponding elements aredenoted by the same symbols throughout the drawings, and duplicatedexplanations are omitted as necessary for the sake of clarity.

Each of the embodiments described below may be used individually, or twoor more of the embodiments may be appropriately combined with oneanother. These embodiments include novel features different from eachother. Accordingly, these embodiments contribute to attaining objects orsolving problems different from one another and contribute to obtainingadvantages different from one another.

The following descriptions on the embodiments mainly focus on 3GPPMulti-RAT Dual Connectivity (MR-DC) using E-UTRA and NR. However, theseembodiments may be applied to other radio communication systemssupporting DC architecture using other different RATs.

First Embodiment

FIG. 6 shows a configuration example of a radio communication networkaccording to a plurality of embodiments including this embodiment. Inthe example shown in FIG. 6, the radio communication network includes amaster node (MN) 1, a secondary node (SN) 2, a UE 3, and a core network4. The radio communication network shown in FIG. 6 supports Multi-RATDual Connectivity (MR-DC). More specifically, one of the MN 1 and the SN2 is an E-UTRA node (i.e., eNB) and the other one is an NR node (i.e.,gNB). At least the MN 1 is connected to the core network 4 via aninterface 601. The SN 2 may also be connected to the core network 4 viaan interface 604. The core network 4 is an EPC in the case of the MRDCwith the EPC, whereas it is a 5GC in the case of the MRDC with the 5GC.The interfaces 601 and 604 are S1 interfaces (i.e., S1-MME and S1-U) inthe case of MRDC with the EPC, whereas they are NG interfaces (i.e.,NG-c and NG-u, or NG2 and NG3) in the case of the MRDC with the 5GC. TheMN 1 and the SN 2 are connected to each other via an interface 603. Theinterface 603 is an X2 interface in the case of the MRDC with the EPC,whereas it is an Xn interface in the case of the MRDC with the 5GC.

The core network 4 includes one or more control plane (CP) nodes 5 andone or more user plane (UP) nodes 6. The CP node 5 may also be referredto as Control Plane Network Functions (CP NFs). The UP node 6 may bereferred to as User Plane Network Functions (UP NFs). In the case of theMRDC with the EPC, for example, one or more CP nodes 5 include an MMEand a Policy and Charging Rules Function (PCRF), while one or more UPnodes 6 include an S-GW and a P-GW. In the case of the MRDC with the5GC, for example, one or more CP nodes 5 include an Access and MobilityManagement Function (AMF), a Session Management Function (SMF), and aPolicy Control function (PCF), while one or more UP nodes 6 include aUser plane Function (UPF).

The UE 3 supports Multi-RAT Dual Connectivity (MR-DC). Specifically, theUE 3 supports a multi-connectivity operation that uses the E-UTRA and NRradio access technologies. In the following description, the RATsupported by the MN 1 is referred to as a master RAT, while the RATsupported by the SN 2 is referred to as a secondary RAT. In other words,the MN 1 and the SN 2 are associated with the master RAT and thesecondary RAT, respectively. In the case of the EN-DC and the NG-EN-DC,the MN 1 is a Master eNB, the SN 2 is a Secondary gNB, the master RAT isE-UTRA, and the secondary RAT is NR (5G RAT). On the other hand, in thecase of the NE-DC, the MN 1 is a Master gNB, the SN 2 is a SecondaryeNB, the master RAT is NR (5G RAT), and the secondary RAT is E-UTRA. TheUE 3 has a capability to communicate simultaneously with the MN 1associated with the master RAT and the SN 2 associated with thesecondary RAT. In other words, the UE 3 has a capability to aggregate acell(s) belonging to the Master Cell Group (MCG) provided by the MN 1with a cell(s) belonging to the Secondary Cell Group (SCG) provided bythe SN 2. The MCG includes one or more cells provided from the masterRAT. The SCG includes one or more cells provided from the secondary RAT.An air interface 602 between the MN 1 and the UE 3 provides a controlplane connection (e.g., RRC connection) and a user plane connection(e.g., user plane bearer). On the other hand, an air interface 605between the gNB 2 and the UE 3 includes at least a user planeconnection, but it does not need to include a control plane connection.

FIG. 7 shows radio protocol architecture for MCG and SCG split bearersaccording to this embodiment. As already described above, the MCG splitbearer is a user plane bearer whose radio protocols are split at the MN1 and belong to both the MCG and the SCG. The SCG split bearer is a userplane bearer whose radio protocols are split at the SN 2 and belong toboth the SCG and the MCG. Although the EN-DC is assumed in this example,the radio protocol architecture for split bearers in the NE-DC and theNG-EN-DC is basically similar to that shown in FIG. 7, except that thereis an SDAP layer that is an upper layer of the unified PDCP layer.

As shown in FIG. 7, in this embodiment, the unified PDCP layer is used.The unified PDCP layer may also be referred to as a single or commonPDCP layer. The unified PDCP layer can be used for both the MCG and SCGsplit bearers. The unified PDCP entities 711 and 721, which are locatedrespectively in the transmitting sides of the MN 1 and the SN 2 as shownin FIG. 7, are PDCP entities in the unified PDCP layer and provideunified PDCP functionalities corresponding to the unified PDCP layer.The unified PDCP functionalities are used for both the MCG and SCG splitbearers. The unified PDCP functionalities or the unified PDCP entities711 and 721 may be implemented in various ways, for example, as shownbelow.

In some implementations, the unified PDCP functionalities may be commonfunctionalities that both the MN PDCP functionalities (e.g., LTE PDCPfunctionalities) that correspond to the master RAT (e.g., E-UTRA) andthe SN PDCP functionalities (e.g., NR PDCP functionalities) thatcorrespond to the secondary RAT (e.g., NR) have. In other words, theunified PDCP functionalities may be a common subset between the MN PDCPfunctionalities (e.g., LTE PDCP functionalities) and the SN PDCPfunctionalities (e.g., NR PDCP functionalities).

In some implementations, the unified PDCP functionalities may beimplemented by executing the MN PDCP functionalities (e.g., NR PDCPfunctionalities), which correspond to the master RAT (e.g., NR), as oneof the modes (or sub-modes) of the SN PDCP functionalities (e.g., LTEPDCP functionalities), which correspond to the secondary RAT (e.g.,LTE).

In some implementations, the unified PDCP functionalities may beimplemented by executing the SN PDCP functionalities (e.g., NR PDCPfunctionalities), which correspond to the secondary RAT (e.g., NR), asone of the modes (or sub-modes) of the MN PDCP functionalities (e.g.,LTE PDCP functionalities), which correspond to the master RAT (e.g.,LTE).

In some implementations, the SN PDCP functionalities (e.g., NR PDCPfunctionalities) may be a subset of the MN PDCP functionalities (e.g.,LTE PDCP functionalities). In this case, the unified PDCPfunctionalities may be the same as, or a subset of, the SN PDCPfunctionalities.

In some implementations, the MN PDCP functionalities (e.g., LTE PDCPfunctionalities) may be a subset of the SN PDCP functionalities (e.g.,NR PDCP functionalities). In this case, the unified PDCP functionalitiesmay be the same as, or a subset of, the MN PDCP functionalities.

In some implementations, the unified PDCP entities 711 and 721 may beimplemented in a manner such that the MN PDCP entity (e.g., LTE PDCPentity) of the MN 1 has a configuration that is, at least in part,common to (or the same as) that for the SN PDCP entity (e.g., NR PDCPentity) of the SN 2.

In some implementations, the unified PDCP entities 711 and 721 may beimplemented in a manner such that the SN PDCP entity (e.g., LTE PDCPentity) of the SN 2 has a configuration that is, at least in part,common to (or the same as) that for the MN PDCP entity (e.g., NR PDCPentity) of the MN 1.

In some implementations, the unified PDCP layer may be implemented in amanner such that the MN PDCP layer (e.g., LTE PDCP layer) performs thefunctionalities of the SN PDCP layer (e.g., NR PDCP layer).

In some implementations, the unified PDCP layer may be implemented in amanner such that the SN PDCP layer (e.g., LTE PDCP layer) performs thefunctionalities of the MN PDCP layer (e.g., NR PDCP layer).

In some implementations, in order to provide the unified PDCPfunctionalities, an SN PDCP (e.g., NR PDCP) layer module operating as anupper (or sub) layer of the MN PDCP (e.g., LTE PDCP) layer may belocated in the MN 1.

In some implementations, in order to provide the unified PDCPfunctionalities, an MN PDCP (e.g., NR PDCP) layer module operating as anupper (or sub) layer of the SN PDCP (e.g., LTE PDCP) layer may belocated in the SN 2.

In some implementations, each of the MeNB 1, the SgNB 2, and the UE 3may prepare a PDCP entity that provides unified PDCP functionalities byestablishing, re-establishing, or reconfiguring a PDCP entity with aconfiguration common to both the MN PDCP functionalities and the SN PDCPfunctionalities, or by switching the mode (or sub-mode) of the PDCPentity.

An MN RLC entity 712, which is an RLC entity located in the MN 1 for theMCG split bearer, receives PDCP PDUs from the unified PDCP entity 711located in the MN 1 and provides RLC PDUs to an MN MAC entity 714. Onthe other hand, an MN RLC entity 713, which is an RLC entity located inthe MN 1 for the SCG split bearer, receives PDCP PDUs from the unifiedPDCP entity 721 located in the SN 2 and provides RLC PDUs to the MN MACentity 714. The MN MAC entity 714 is a MAC entity located in the MN 1for the UE 3. The MN RLC entities 712 and 713 and the MN MAC entity 714provide RLC functions and MAC functions conforming to the master RAT(e.g., E-UTRA).

An SN RLC entity 722, which is an RLC entity located in the SN 2 for theSCG split bearer, receives PDCP PDUs from the unified PDCP entity 721located in the SN 2 and provides RLC PDUs to an SN MAC entity 724. Onthe other hand, the SN RLC entity 723, which is an RLC entity located inthe SN 2 for the MCG split bearer, receives PDCP PDUs from the unifiedPDCP entity 711 located in the MN 1 and provides RLC PDUs to the SN MACentity 724. The SN MAC entity 724 is a MAC entity located in the SN 2for the UE 3. The SN RLC entities 722 and 723 and the SN MAC entity 724provide RLC functions and MAC functions conforming to the secondary RAT(e.g., NR).

Unified PDCP entities 731 and 735, which are located in the receivingside of the UE 3 as shown in FIG. 7, are PDCP entities in the unifiedPDCP layer and provide unified PDCP functionalities corresponding to theunified PDCP layer. The unified PDCP entities 731 and 735 may beimplemented in a way similar to that for the above-described unifiedPDCP entities 711 and 721. The method of implementing the unified PDCPentities 731 and 735 in the UE 3 may be different from the method ofimplementing the unified PDCP entities 711 and 721 in the MN 1 and theSN 2.

An MN MAC entity 734 is a MAC entity located in the UE 3 forcommunicating with the MN 1 via an MCG cell. The MN MAC entity 734receives MAC PDUs from a lower layer (i.e., physical layer) (not shown)and provides MAC SDUs (RLC PDUs) to MN RLC entities 732 and 733. The MNRLC entity 732, which is an RLC entity located in the UE 3 for the MCGsplit bearer, provides RLC SDUs (PDCP PDUs) to the unified PDCP entity731. On the other hand, the MN RLC entity 733, which is an RLC entitylocated in the UE 3 for the SCG split bearer, provides RLC SDUs (PDCPPDUs) to the unified PDCP entity 735. The MN RLC entities 732 and 733and the MN MAC entity 734 provide RLC functions and MAC functionsconforming to the master RAT (e.g., E-UTRA).

An SN MAC entity 738 is a MAC entity located in the UE 3 forcommunicating with the SN 2 via an SCG cell. The SN MAC entity 738receives MAC PDUs from a lower layer (i.e., physical layer) (not shown)and provides MAC SDUs (RLC PDUs) to SN RLC entities 736 and 737. The SNRLC entity 736, which is an RLC entity located in the UE 3 for the SCGsplit bearer, provides RLC SDUs (PDCP PDUs) to the unified PDCP entity735. On the other hand, the SN RLC entity 737, which is an RLC entitylocated in the UE 3 for the MCG split bearer, provides RLC SDUs (PDCPPDUs) to the unified PDCP entity 731. The SN RLC entities 736 and 737and the SN MAC entity 738 provide RLC functions and MAC functionsconforming to the secondary RAT (e.g., NR).

In the following, operations of the MN 1, the SN 2, and the UE 3according to this embodiment will be described. The MN 1 is configuredto communicate with the SN 2 associated with the secondary RAT andprovide the UE 3 with dual connectivity that uses the master RAT and thesecondary RAT. The MN 1 is further configured to, in response toreceiving, from the UE 3 or the core network 4, UE capabilityinformation indicating that the UE 3 supports split bearers, configure(or establish) the PDCP entity 711 providing the unified PDCPfunctionalities and use this PDCP entity 711 for an MCG split bearer forthe UE 3. In addition, in order to configure the PDCP entity 731 in theUE 3 to provide the unified PDCP functionalities, the MN 1 transmitsunified PDCP configuration information (e.g., unified PDCP-config)regarding the MCG split bearer to the UE 3. The UE 3 receives theunified PDCP configuration information regarding the MCG split bearerand configures (or establishes) the unified PDCP entity 731 for the MCGsplit bearer in accordance with this information. Accordingly, the UE 3uses the unified PDCP entity 731, which provides the unified PDCPfunctionalities, for the MCG split bearer for the UE 3.

The SN 2 is configured to communicate the MN 1 associated with themaster RAT and provide the UE 3 with dual connectivity that uses themaster RAT and the secondary RAT. The SN 2 is further configured to, ifthe MN 1 has received from the UE 3 or the core network 4 the UEcapability information indicating that the UE 3 supports split bearers,configure (or establish) the PDCP entity 721 providing the unified PDCPfunctionalities and use this PDCP entity 721 for an SCG split bearer forthe UE 3. In addition, in order to configure the PDCP entity 735 in theUE 3 to provide the unified PDCP functionalities, the SN 2 transmitsunified PDCP configuration information (e.g., unified PDCP-config)regarding the SCG split bearer to the UE 3. This PDCP configurationinformation may be sent to the UE 3 via the MN 1, or may be sentdirectly from the SN 2 to the UE 3 if an RRC connection between the SN 2and the UE 3 is available. The UE 3 receives the unified PDCPconfiguration information regarding the SCG split bearer and configures(or establishes) the unified PDCP entity 735 for the SCG split bearer inaccordance with this information. Accordingly, the UE 3 uses the unifiedPDCP entity 735, which provides the unified PDCP functionalities, forthe SCG split bearer for the UE 3.

In the above example, it is assumed that the UE capability is specifiedon the premise that the UE 3 supporting the unified PDCP functionalitiesalways supports both MCG and SCG split bearers. In other words, it isassumed that one UE capability regarding the support of split bearersindicates support of both MCG and SCG split bearers, and that the UE 3 tsupporting split bearers supports the unified PDCP functionalities.Alternatively, separate UE capabilities may be specified respectivelyfor MCG split bearers and SCG split bearers. In other words, discrete UEcapabilities may be specified respectively for MCG and SCG splitbearers, and the UE 3 that supports at least one of the two types ofsplit bearers may support the unified PDCP functionalities.

Further, the UE capability information may explicitly or implicitlyindicate that the UE 3 supports split bearers in the MR-DC. The UEcapability information may be, for example, information (e.g., unifiedbearer support) indicating whether the UE 3 supports unified bearers.Alternatively, the UE capability information may be information (e.g.,unified PDCP support) indicating whether the UE 3 supports unified PDCP.Alternatively, the UE capability information may be information (e.g.,EN-DC support, NG-EN-DC support, NE-DC support) indicating whether theUE 3 supports MR-DC (i.e., EN-DC, NG-EN-DC, NE-DC, or any combinationthereof).

FIG. 8 is a sequence diagram showing one example of a procedure forestablishing an MCG split bearer according to this embodiment. FIG. 8shows an example of the EN-DC. Specifically, in FIG. 8, the MN 1 is aMaster eNB (MeNB), the SN 2 is a Secondary gNB (SgNB), and the corenetwork 4 is an EPC. In Step 801, the UE 3 establishes an RRC connectionwith the MeNB 1, establishes a Non-Access Stratum (NAS) connection withthe EPC 4 (e.g., an MME serving as the CP node 5) via the MeNB 1, andestablishes an MCG bearer in an MCG cell provided by the MeNB 1.

Further, in Step 801, the MeNB 1 receives UE capability information fromthe UE 3 or the EPC 4 (e.g., an MME serving as the CP node 5). This UEcapability information explicitly or implicitly indicates that the UE 3supports split bearers.

In Step 802, in order to configure an MCG split bearer for the UE 3, theMeNB 1 sends an SgNB Addition (or Modification) Request message to theSgNB 2 via the interface 603 (i.e., X2 interface). This SgNB Addition(or Modification) Request message contains a Bearer Option InformationElement (IE) set to the value “MCG split bearer”. This SgNB Addition (orModification) Request message also contains an RRC container containingan SCG-ConfigInfo message. This SCG-ConfigInfo message includesconfigurations for the MCG split bearer and also includes a drb-typeinformation element (IE) set to the value “MCG split”.

In Step 803, the SgNB 2 sends an SgNB Addition (or Modification) RequestAcknowledge message to the MeNB 1 via the interface 603 (i.e., X2interface). This message contains an RRC container containing anSCG-Config message. This SCG-Config message includes SCG configurationsfor the MCG split bearer and also includes a drb-type informationelement (IE) set to the value “MCG split”.

In Step 804, the MeNB 1 transmits an RRC Connection Reconfigurationmessage to the UE 3. This message contains unified PDCP configurationinformation (e.g., unified PDCP-config) regarding the MCG split bearer.This unified PDCP configuration information causes the UE 3 toestablish, re-establish, or configure a unified PDCP entity for the MCGsplit bearer. This message also contains SCG configurations includingthe SCG-Config information element (IE) and including other informationfor the NR SCG. The change of the bearer type from the MCG bearer to theMCG split bearer may be implicitly indicated by the value of thedrb-type IE within the SCG-Config IE. Alternatively, the RRC ConnectionReconfiguration message may explicitly indicate the change of the bearertype from the MCG bearer to the MCG split bearer.

In Step 805, the UE 3 prepares a unified PDCP for the MCG split bearer.Similarly, in Step 806, the MeNB 1 prepares a unified PDCP for the MCGsplit bearer. The unified PDCP preparation by the UE 3 in Step 805 mayinclude the following processing. The unified PDCP preparation by theMeNB 1 in Step 806 may also include similar processing.

When an MCG split bearer is directly configured, that is, when a newbearer (DRB) is originally established as an MCG split bearer, the UE 3newly establishes, for the MCG split bearer, the PDCP entity 731 in theunified PDCP layer to provide the unified PDCP functionalities. The MeNB1 receives from the SgNB 2, during the SgNB Addition procedure or theSgNB Modification procedure, information (e.g., SCG-Config) necessary togenerate SCG configurations (Step 803), and transmits information forDRB addition (i.e., DrbToAddMod: drb-type: MCG split) to the UE 3 (Step804).

When a MCG bearer is changed to an MCG split bearer, the UE 3re-establishes a PDCP entity for the MCG bearer as a unified PDCPentity. The UE 3 may re-establish the unified PDCP entity by applyingthe unified PDCP configuration (unified PDCP-config) while reusing apart of the PDCP configuration (i.e., LTE PDCP-config) for the MCGbearer. The UE 3 may re-establish the unified PDCP entity by applying anew unified PDCP configuration.

Alternatively, when an MCG bearer is changed to an MCG split bearer, theUE 3 may reconfigure a PDCP entity for the MCG bearer as a unified PDCPentity. Alternatively, the UE 3 may switch the operation mode of thePDCP entity for the MCG bearer to the (sub) mode corresponding to theunified PDCP. The UE 3 may reconfigure the PDCP entity by applyingadditional PDCP configuration (unified PDCP-config) necessary to providethe unified PDCP functionalities while reusing a part of the PDCPconfiguration (i.e., LTE PDCP-config) for the MCG bearer.

That is, the unified PDCP configuration information (unifiedPDCP-config) transmitted from the MN 1 to the UE 3 may be new PDCPconfiguration information for the MCG split bearer (i.e., full-config).In addition, or alternatively, the unified PDCP configurationinformation (unified PDCP-config) transmitted from the MN 1 to the UE 3may include PDCP configuration information to be added to the PDCPconfiguration (i.e., LTE PDCP-config) for the MCG bearer (i.e.,delta-config), or to be deleted from the PDCP configuration for the MCGbearer, in order to configure the unified PDCP entity.

Referring back to FIG. 8, the UE 3 transmits an RRC ConnectionReconfiguration Complete message to the MeNB 1 in Step 807. In Step 808,the MeNB 1 sends an SgNB Addition Complete message to the SgNB 2. InStep 809, the UE 3 performs a random access procedure to the SgNB 2.Accordingly, the UE 3 is able to receive user plane (UP) data via theMCG split bearer (Step 810).

FIG. 9 is a sequence diagram showing one example of a procedure forestablishing an SCG split bearer according to this embodiment. FIG. 9shows an example of the EN-DC. Specifically, in FIG. 9, the MN 1 is aMaster eNB (MeNB), the SN 2 is a Secondary gNB (SgNB), and the corenetwork 4 is an EPC. The processing of Step 901 is similar to theprocessing of Step 801. In Step 901, the MeNB 1 receives UE capabilityinformation from the UE 3 or the EPC 4 (e.g., an MME serving as the CPnode 5). This UE capability information explicitly or implicitlyindicates that the UE 3 supports split bearers.

In Step 902, in order to configure an SCG split bearer for the UE 3, theMeNB 1 sends an SgNB Addition (or Modification) Request message to theSgNB 2 via the interface 603 (i.e., X2 interface). This SgNB Addition(or Modification) Request message contains a Bearer Option InformationElement (IE) set to the value “SCG split bearer”. This SgNB Addition (orModification) Request message also contains an RRC container containingan SCG-ConfigInfo message. This SCG-ConfigInfo message includesconfigurations for the SCG split bearer and also includes a drb-typeinformation element (IE) set to the value “SCG split”.

In Step 903, the SgNB 2 sends an SgNB Addition (or Modification) RequestAcknowledge message to the MeNB 1 via the interface 603 (i.e., X2interface). This message contains an RRC container containing anSCG-Config message. This SCG-Config message includes SCG configurationsfor the SCG split bearer and also includes a drb-type informationelement (IE) set to the value “SCG split”. These SCG configurationsinclude unified PDCP configuration information (e.g., unifiedPDCP-config) regarding the SCG split bearer. The SCG configurations mayinclude, for example, a DRB-ToAddModSCG IE containing a drb-type IE setto the value “scg-split” and also containing a pdcp-Config IE indicatingthe unified PDCP configuration.

In Step 904, the MeNB 1 transmits an RRC Connection Reconfigurationmessage to the UE 3. This message contains SCG configurations includingthe SCG-Config information element (IE) and including other informationfor the NR SCG. These SCG configurations include the unified PDCPconfiguration information (e.g., unified PDCP-config) regarding the SCGsplit bearer. This unified PDCP configuration information causes the UE3 to establish, re-establish, or configure a unified PDCP entity for theSCG split bearer. The other information for the NR SCG includes, forexample, SCG Security. The change of the bearer type from the MCG beareror the SCG bearer to the SCG split bearer may be implicitly indicated bythe value of the drb-type IE within the SCG-Config IE. Alternatively,the RRC Connection Reconfiguration message may explicitly indicate thechange of the bearer type from the MCG bearer or the SCG bearer to theSCG split bearer.

In Step 905, the UE 3 prepares a unified PDCP for the SCG split bearer.Similarly, in Step 906, the SgNB 2 prepares a unified PDCP for the SCGsplit bearer. The unified PDCP preparation by the UE 3 in Step 905 mayinclude the following processing. The unified PDCP preparation by theSgNB 2 in Step 906 may also include similar processing.

When an MCG bearer is changed to an SCG split bearer (e.g., SgNBAddition procedure), the UE 3 newly establishes, for the SCG splitbearer, the PDCP entity 735 in the unified PDCP layer to provide theunified PDCP functionalities. The UE 3 may release a PDCP entity usedfor the MCG bearer. Alternatively, the UE 3 may maintain the PDCP entityused for the MCG bearer. The maintained PDCP entity may be used forforwarding of a flow(s) (PDU flow(s), QoS flow(s)) that have not beenmoved from the MCG bearer to the SCG split bearer.

When an SCG bearer is changed to an SCG split bearer (e.g., SgNBModification procedure), the UE 3 re-establishes a PDCP entity for theSCG bearer as a unified PDCP. The UE 3 may re-establish the unified PDCPentity by applying the unified PDCP configuration (unified PDCP-config)while reusing a part of the PDCP configuration (i.e., NR PDCP-config)for the SCG bearer. The UE 3 may re-establish the unified PDCP entity byapplying a new unified PDCP configuration.

Alternatively, when an SCG bearer is changed to an SCG split bearer, theUE 3 may reconfigure a PDCP entity for the SCG bearer as a unified PDCP.Alternatively, the UE 3 may switch the operation mode of the PDCP entityfor the SCG bearer to the (sub) mode corresponding to the unified PDCP.The UE 3 may reconfigure the PDCP entity by applying additional PDCPconfiguration (unified PDCP-config) necessary to provide the unifiedPDCP functionalities while reusing a part of the PDCP configuration(i.e., NR PDCP-config) for the SCG bearer.

Referring back to FIG. 9, the UE 3 transmits an RRC ConnectionReconfiguration Complete message to the MeNB 1 in Step 907. In Step 908,the MeNB 1 sends an SgNB Addition Complete message to the SgNB 2. InStep 909, the UE 3 performs a random access procedure to the SgNB 2.Accordingly, the UE 3 is able to receive user plane (UP) data via theSCG split bearer (Step 910).

Second Embodiment

This embodiment provides a modified example of the implementation of theunified PDCP layer in the radio communication network described in thefirst embodiment. A configuration example of a radio communicationnetwork according to this embodiment is similar to the example shown inFIG. 6. Radio protocol architecture for MCG and SCG split bearersaccording to this embodiment is similar to the example shown in FIG. 7.

In this embodiment, an MN 1 and a UE 3 are configured to, when theystarts the MR-DC, use a unified PDCP entity for a newly configured MCGsplit bearer, SCG bearer, or SCG split bearer. The MN 1 and the UE 3 arefurther configured to, when they starts the MR-DC, use the unified PDCPentity, which provides the unified PDCP functionalities, also for analready established MCG bearer for the UE 3. In other words, when the UE3 supporting split bearers in MR-DC starts the MR-DC, the MN 1 and theUE 3 use a unified PDCP entity also for an MCG bearer for the UE 3,regardless of whether an MCG split bearer is used for the UE 3.

When the MN 1 starts the MR-DC with the UE 3, the MN 1 may newly (again)establish a unified PDCP entity as the PDCP entity for the alreadyestablished MCG bearer of the UE 3. The MN 1 may reuse a part of thePDCP configuration (e.g., LTE PDCP-config) or DRB configuration (e.g.,LTE DRB config) of the MCG bearer.

Alternatively, when the MN 1 starts MR-DC with the UE 3, the MN 1 mayre-establish the PDCP entity for the already established MCG bearer ofthe UE 3 in such a way that it provides the unified PDCPfunctionalities. The MN 1 may re-establish the PDCP entity for the MCGbearer by applying the unified PDCP configuration (unified PDCP-config)while re-using a part of the PDCP configuration (e.g., LTE PDCP-config)of the MCG bearer. The MN 1 may re-establish the PDCP entity for the MCGbearer by applying the new unified PDCP configuration.

Alternatively, when the MN 1 starts MR-DC with the UE 3, the MN 1 mayreconfigure the PDCP entity for the already established MCG bearer ofthe UE 3 in such a way that it provides the unified PDCPfunctionalities. Alternatively, the UE 3 may switch the operation modeof the PDCP entity of the already established MCG bearer to the (sub)mode corresponding to the unified PDCP. The MN 1 may reconfigure thePDCP entity for the MCG bearer by applying additional PDCP configuration(unified PDCP-config) for providing the unified PDCP functionalitieswhile re-using a part of the PDCP configuration (e.g., LTE PDCP-config)of the MCG bearer.

In a similar way, in this embodiment, the UE 3 is configured to, whenthe UE 3 starts MR-DC, use the unified PDCP entity for a newlyconfigured MCG split bearer, SCG bearer, or SCG split bearer. The UE 3is further configured to, when the UE 3 starts MR-DC, use the unifiedPDCP entity, which provides the unified PDCP functionalities, also forthe already established MCG bearer. In other words, when the UE 3supporting split bearers starts the MR-DC, this UE 3 uses a unified PDCPentity also for an MCG bearer, regardless of whether an MCG split beareris used for this UE 3.

FIG. 10 is a sequence diagram showing one example of an MR-DC startingprocedure involving establishment of an MCG split bearer according tothis embodiment. FIG. 10 shows an example of the EN-DC. Specifically, inFIG. 10, the MN 1 is a Master eNB (MeNB), the SN 2 is a Secondary gNB(SgNB), and the core network 4 is an EPC. The processing of Step 1001 issimilar to the processing of Step 801 in FIG. 8. In Step 1001, the MeNB1 receives UE capability information from the UE 3 or the EPC 4 (e.g.,an MME serving as the CP node 5). This UE capability informationexplicitly or implicitly indicates that the UE 3 supports split bearers.

In Step 1002, in order to start the EN-DC with the UE 3, the MeNB 1sends an SgNB Addition Request message to the SgNB 2 via the interface603 (i.e., X2 interface). This SgNB Addition Request message containsinformation elements (IEs) similar to those contained in the SgNBAddition (or Modification) Request message of Step 802 in FIG. 8.

In Step 1003, the SgNB 2 sends an SgNB Addition Request Acknowledgemessage to the MeNB 1 via the interface 603 (i.e., X2 interface). ThisgNB Addition Request Acknowledge message contains information elements(IEs) similar to those contained in the SgNB Addition (or Modification)Request Acknowledge message of Step 803 in FIG. 8.

The processing of Steps 1004-1010 is similar to the processing of Steps804-810 in FIG. 8. However, the RRC Connection Reconfiguration messageof Step 1004 further includes unified PDCP configuration informationregarding an MCG bearer. For example, this RRC ConnectionReconfiguration message may include a DRB-ToAddModList IE indicatingaddition and deletion of MCG bearers, or indicating modification of MCGbearers, and the DRB-ToAddModList IE may also include a DRB-ToAddMod IEthat contains a pdcp-Config IE indicating the unified PDCPconfiguration. In Step 1005, as well as the preparation of a unifiedPDCP entity for an MCG split bearer, the UE 3 establishes,re-establishes, or reconfigures a PDCP entity for an already establishedMCG bearer in order to apply the unified PDCP also to this MCG bearer.Alternatively, the UE 3 may switch the operation mode of the PDCP entityof the already established MCG bearer to the (sub) mode corresponding tothe unified PDCP. In a similar way, in Step 1006, as well as thepreparation of a unified PDCP entity for an MCG split bearer, the MeNB 1establishes, re-establishes, or reconfigures a PDCP entity for an MCGbearer already established for the UE 3 in order to apply the unifiedPDCP also to this MCG bearer. Alternatively, the MeNB 1 may switch theoperation mode of the PDCP entity of the MCG bearer already establishedfor the UE 3 to the (sub) mode corresponding to the unified PDCP.

FIG. 11 is a sequence diagram showing one example of an MR-DC startingprocedure involving establishment of an SCG bearer or an SCG splitbearer according to this embodiment. FIG. 11 shows an example of theEN-DC. Specifically, in FIG. 11, the MN 1 is a Master eNB (MeNB), the SN2 is a Secondary gNB (SgNB), and the core network 4 is an EPC. Theprocessing of Step 1101 is similar to the processing of Step 1101 inFIG. 8. In Step 1101, the MeNB 1 receives UE capability information fromthe UE 3 or the EPC 4 (e.g., an MME serving as the CP node 5). This UEcapability information explicitly or implicitly indicates that the UE 3supports split bearers.

In Step 1102, in order to start the EN-DC with the UE 3, the MeNB 1sends an SgNB Addition Request message to the SgNB 2 via the interface603 (i.e., X2 interface). This SgNB Addition Request message contains aBearer Option information element (IE) set to the value “SCG bearer” or“SCG split bearer”. This SgNB Addition (or Modification) Request messagealso contains an RRC container including an SCG-ConfigInfo message. ThisSCG-ConfigInfo message includes configurations for an SCG split bearerand also includes a drb-type information element (IE) set to the value“SCG” or “SCG split”.

In Step 1103, the SgNB 2 sends an SgNB Addition Request Acknowledgemessage to the MeNB 1 via the interface 603 (i.e., X2 interface). Thismessage contains an RRC container containing an SCG-Config message. ThisSCG-Config message includes SCG configurations for an SCG split bearerand also includes a drb-type information element (IE) set to the value“SCG” or “SCG split”.

The processing of Steps 1104-1110 is similar to the processing of Steps904-910 in FIG. 9. However, the RRC Connection Reconfiguration messageof Step 1104 further includes unified PDCP configuration informationregarding an MCG bearer. For example, this RRC ConnectionReconfiguration message may include a DRB-ToAddModList IE indicatingaddition and deletion of MCG bearers, or modification of MCG bearers,and the DRB-ToAddModList IE may include a DRB-ToAddMod IE that containsa pdcp-Config IE indicating the unified PDCP configuration. In Step1105, as well as the preparation of a unified PDCP entity for an SCGbearer or an SCG split bearer, the UE 3 establishes, re-establishes, orreconfigures a PDCP entity for an already established MCG bearer, orswitch the (sub) mode of the PDCP entity for this MCG bearer, in orderto apply the unified PDCP also to this MCG bearer. In Step 1106, theSgNB 2 prepares the unified PDCP for the SCG split bearer or the SCGbearer.

The procedure shown in FIG. 11 further includes Step 1106B. In Step1006B, the MeNB 1 establishes, re-establishes, or reconfigures a PDCPentity for an MCG bearer already established for the UE 3, or switchesthe (sub) mode of the PDCP entity for this MCG bearer, in order to applythe unified PDCP also to this MCG bearer.

As described above, when starting the MR-DC, the radio communicationsystem according to this embodiment applies the unified PDCP to otherbearers, such as an already established MCG bearer, as well as applyingto the split bearer. Accordingly, it is possible to reduce a processingdelay in a bearer type change to a split bearer from another bearer type(MCG bearer, SCG bearer) or a bearer type change from a split bearer toanother bearer type. The processing delay here includes, for example,re-establishment of at least one of a PDCP entity and an RLC entity, orreset of a MAC entity, or both.

Third Embodiment

This embodiment provides a modified example of the implementation of theunified PDCP layer in the radio communication network described in thesecond embodiment. A configuration example of a radio communicationnetwork according to this embodiment is similar to the example shown inFIG. 6. Radio protocol architecture for MCG and SCG split bearersaccording to this embodiment is similar to the example shown in FIG. 7.

In this embodiment, the SN 2 and the UE 3 are configured to, whenestablishing for an SCG split bearer a unified PDCP entity providing theunified PDCP functionalities, establish, re-establish, or reconfigure aPDCP entity for an already established SCG bearer, or switch the (sub)mode of the PDCP entity for this SCG bearer, in such a way that itprovides the unified PDCP functionalities.

Fourth Embodiment

This embodiment provides a modified example of the implementation of theunified PDCP layer in the radio communication network described in thefirst and second embodiments. A configuration example of a radiocommunication network according to this embodiment is similar to theexample shown in FIG. 6. Radio protocol architecture for MCG and SCGsplit bearers according to this embodiment is similar to the exampleshown in FIG. 7.

In this embodiment, the MN 1 is configured to, if the UE 3 does notsupport split bearers in the MR-DC, use, for an MCG bearer for the UE 3,a PDCP entity that provides the MN PDCP functionalities corresponding tothe master RAT. The MN 1 is further configured to, if the UE 3 supportssplit bearers in the MR-DC, use, for an MCG bearer for the UE 3, aunified PDCP entity that provides the unified PDCP functionalitiesregardless of whether the MR-DC is started for the UE 3. In other words,the MN 1 is configured to, if the UE 3 supports split bearers in theMR-DC, use a unified PDCP entity for an MCG bearer for the UE 3 beforethe MR-DC is started for the UE 3.

The UE 3 is configured to, if the UE 3 does not support split bearers inthe MR-DC, use, for an MCG bearer for the UE 3, a PDCP entity thatprovides the MN PDCP functionalities corresponding to the master RAT.The UE 3 is further configured to, if the UE 3 supports split bearers inthe MR-DC, use, for an MCG bearer for the UE 3, a unified PDCP entitythat provides the unified PDCP functionalities regardless of whether theMR-DC is started for the UE 3. In other words, the UE 3 is configuredto, if the UE 3 supports split bearers in the MR-DC, use a unified PDCPentity for an MCG bearer for the UE 3 before the MR-DC is started forthe UE 3.

FIG. 12 is a sequence diagram showing one example of a procedure forestablishing an RRC connection and a user plane bearer according to thisembodiment. FIG. 12 shows an example of the EN-DC. Specifically, in FIG.12, the MN 1 is a Master eNB (MeNB), the SN 2 is a Secondary gNB (SgNB),and the core network 4 is an EPC.

Steps 1201-1203 show an RRC connection establishment procedure. In Step1201, the UE 3 transmits an RRC Connection Request message to the MeNB1. In Step 1202, the MeNB 1 transmits an RRC Connection Setup message tothe UE 3. In Step 1203, the UE 3 transmits an RRC Connection SetupComplete message to the MeNB 1. The RRC Connection Setup Completemessage includes an initial NAS message (e.g., Service Request message)from the UE 3 to the EPC 4.

In Step 1204, the MeNB 1 sends to the EPC 4 an INITIAL UE MESSAGEmessage that contains the initial NAS message received from the UE 3. InStep 1205, the EPC 4 (e.g., an MME serving as the CP node 5) sends anINITIAL CONTEXT SETUP REQUEST message to the MeNB 1. This INITIALCONTEXT SETUP REQUEST message contains UE radio access capabilityinformation including a UE capability information element indicatingthat the UE 3 supports split bearers (e.g., “Split Bearer Support” or“Unified Bearer Support”). The name of the UE capability informationelement (IE) indicating the support of split bearers in the MR-DC may be“Unified PDCP Support”, “EN-DC Support”, “NG-EN-DC Support”, or “NE-DCSuppoort”.

In Step 1206, the MeNB 1 performs Access Stratum (AS) securityactivation with the UE 3.

In Step 1207, the MeNB 1 transmits an RRC Connection Reconfigurationmessage to the UE 3. This RRC Connection Reconfiguration messagecontains DRB configuration to be applied to an MCG bearer. This PDCPconfiguration (PDCP-Config) contains PDCP configuration (PDCP-Config) touse the unified PDCP for the MCG bearer.

In Step 1208, the UE 3 prepares a unified PDCP for the MCG bearer. In asimilar way, in Step 1209, the MeNB 1 prepares a unified PDCP for theMCG bearer. Specifically, each of the MeNB 1 and the UE 3 newlyestablishes a unified PDCP entity as the PDCP entity for the MCG bearer.

In Step 1210, the UE 3 transmits an RRC Connection ReconfigurationComplete message to the MeNB 1. In Step 1211, the MeNB 1 sends anINITIAL CONTEXT SETUP RESPONSE message to the EPC 4.

In Step 1212, establishment of a new (MCG) bearer based on a NASExtended Service Request message may be performed.

In Step 1213, an SgNB Addition procedure for starting the MR-DC (i.e.,EN-DC in this example) is performed. This SgNB Addition procedure may beperformed in accordance with, for example, one of the specific examples(FIG. 8 or 9) described in the first embodiment.

In the procedure shown in FIG. 12, the EPC 4 (e.g., an MME serving asthe CP node 5) sends to the MeNB 1 the UE capability informationindicating that the UE 3 supports split bearers. Alternatively, the UE 3may transmit this UE capability information to the MeNB 1.

In some implementations, the UE 3 may transmit the UE capabilityinformation to the MeNB 1 during the RRC connection establishmentprocedure (Steps 1201-1203).

In some implementations, the UE 3 may transmit the UE capabilityinformation to the MeNB 1 using the RRC Connection Request message (Step1201). Accordingly, the MeNB 1 is able to know whether the UE 3 supportssplit bearers prior to the establishment of the RRC connection. Thus,for example, the MeNB 1 is able to use the unified PDCP for a PDCPconfiguration of a signalling radio bearer (SRB 1) for transferring RRCmessages. Specifically, the MeNB 1 may transmit to the UE 3 an RRCConnection Setup message (Step 1202) that contains informationindicating use of the unified PDCP (Unified PDCP indication) or containsa PDCP configuration corresponding to the unified PDCP functionalities.

In some implementations, the UE 3 may transmit the UE capabilityinformation to the MeNB 1 using a third message (i.e., Message 3 (Msg3))in a random access procedure, instead of using the RRC ConnectionRequest message. The third message in the random access procedurecarrying the UE capability information may be, for example, an RRCConnection Re-establishment Request message, an RRC Connection ResumeRequest message, or an RRC Connection

Activation Request message. The RRC Connection Activation Requestmessage is a message transmitted by a UE to request transition from anRRC_INACTIVE state (which is newly introduced in 5G) to an RRC_CONNECTEDstate.

The UE capability information sent from the UE 3 to the MeNB 1 duringthe RRC connection establishment procedure may be referred to as “EarlyUE Capability Indication”. This UE capability information may be definedto be an RRC information element (IE). This RRC IE may be called, forexample, a “splitBearer (Support) IE”, a “unifiedBearer (Support) IE”,or a “unifiedPDCP (Support) IE”. This UE capability information may bean Inter-Operability Test (IOT) bit. This IOT bit is a flag indicatingcompletion of an Inter-Operability Test. Alternatively, an RRC IE (e.g.,earlyCapabilitylndication IE) for Early UE Capability Indication to beused for multiple purposes may be defined, and the information regardingthe MR-DC may be collected in a single field (e.g., MultiRAT-DC, MR-DC)within this RRC IE. Then the UE capability information may be sent usinga subfield (e.g., splitBearer subfield or unified PDCP subfield)included in the field regarding the MR-DC.

Alternatively, this UE capability information (i.e., Early UE CapabilityIndication) may be defined to be a MAC Control Element (CE). This MAC CEmay be called, for example, a “Split Bearer (Support) MAC CE”, a“Unified Bearer (Support) MAC CE”, or a “Unified PDCP (Support) MAC CE”.Alternatively, a MAC CE (e.g., Early Capability Indication MAC CE) forEarly UE Capability Indication to be used for multiple purposes may bedefined, and information regarding the MR-DC including the Early UECapability Indication may be sent using a bitmap in this MAC CE.

Alternatively, this UE capability information (i.e., Early UE CapabilityIndication) may be defined to be a Logical Channel ID (LCID) of anUplink (UL) Common Control Channel (CCCH) (i.e., UL LCID for CCCH(SRBO)). This may be defined to be a new LCID different from LCIDs forother CCCHs, in order to indicate the support of split bearers. When theUE 3 supports split bearers, the UE 3 may transmit a third message(e.g., RRC Connection Request message) using this new LCID.

According to the above methods, the network (e.g., the MN, or the SN 2,or both) is able to know at an early stage of the RRC connectionestablishment that the UE 3 supports split bearers in the MR-DC and theunified PDCP therefor. By using the unified PDCP from the stage of newbearer establishment, it is for example possible to omit a processingdelay, such as switch between the LTE PDCP and the unified PDCP.

FIG. 13 is a sequence diagram showing an example in which the aboveEarly UE Capability Indication is used. FIG. 13 shows an example of theEN-DC. Specifically, in FIG. 13, the MN 1 is a Master eNB (MeNB), the SN2 is a Secondary gNB (SgNB), and the core network 4 is an EPC.

Steps 1301-1303 show an RRC connection establishment procedure. In Step1301, the UE 3 transmits to the MeNB 1 an RRC Connection Request messagecontaining the Early UE Capability Indication indicating the support ofsplit bearers. In Step 1302, the MeNB 1 transmits an RRC ConnectionSetup message to the UE 3. This RRC Connection Setup message containsinformation indicating use of the unified PDCP (Unified PDCPindication), or contains a PDCP configuration corresponding to theunified PDCP functionalities. In Step 1303, the UE 3 transmits an RRCConnection Setup Complete message to the MeNB 1. The RRC ConnectionSetup Complete message includes an initial NAS message (e.g., ServiceRequest message) from the UE 3 to the EPC 4.

In Step 1304, the MeNB 1 sends to the EPC 4 an INITIAL UE MESSAGEmessage that contains the initial NAS message received from the UE 3. InStep 1305, the EPC 4 sends an INITIAL CONTEXT SETUP REQUEST message tothe MeNB 1.

In Step 1306, the UE 3 prepares the unified PDCP for a signalling radiobearer (SRB 1) for transferring RRC messages, according to theprocessing that will be specified in the specification for the unifiedPDCP, or according to the PDCP configuration received in Step 1302. In asimilar way, in Step 1307, the MeNB 1 prepares the unified PDCP for thesignalling radio bearer (SRB 1). That is, each of the MeNB 1 and the UE3 newly establishes a unified PDCP entity as a PDCP entity for thesignalling radio bearer (SRB 1).

In Step 1308, the MeNB 1 performs AS security activation with the UE 3.This AS security activation is performed via the signalling radio bearer(SRB 1), in which the unified PDCP is used, or to which the PDCPconfiguration corresponding to the unified PDCP functionalities isapplied.

In Step 1309, the MeNB 1 sends an RRC Connection Reconfiguration messageto the UE 3. This RRC Connection Reconfiguration message contains a DRBconfiguration to be applied to an MCG bearer. This PDCP configuration(PDCP-Config) contains PDCP configuration (PDCP-Config) to use theunified PDCP for the MCG bearer.

The processing of Steps 1310-1313 is similar to the processing of Steps1210-1213 shown in FIG. 12.

The UE 3 may perform transmission of the Early UE Capability Indicationdescribed above, only under a certain condition. The certain conditionmay be, for example, that information indicating support of the MR-DC(or split bearers in the MR-DC) or the unified PDCP by the MN 1 (i.e.,serving RAN node, e.g., eNB or gNB) is broadcast in the serving cell(e.g., PCell) of the UE 3. The certain condition may be that informationindicating that the MN 1 allows transmission of the Early UE CapabilityIndication is broadcast in the serving cell of the UE 3. Alternatively,the MN 1 may request the UE 3 for the Early UE Capability Indication viaa Message 4 (e.g., RRC Connection Setup) in the random access procedureand the UE 3 may transmit the Early UE Capability Indication via aMessage 5 (e.g., RRC Connection Setup Complete) in response to therequest.

Fifth Embodiment

This embodiment provides specific examples of the unified PDCPconfiguration described in the first to fourth embodiments. Aconfiguration example of a radio communication network according to thisembodiment is similar to the example shown in FIG. 6. Radio protocolarchitecture for MCG and SCG split bearers according to this embodimentis similar to the example shown in FIG. 7.

As one example, the LTE PDCP-config may include at least one of:discardTimer; rlc-AM; rlc-UM; headerCompression; rn-IntegrityProtection;pdcp-SN-Size; ul-DataSplitDRB-ViaSCG; and t-Reordering.

The discardTimer field indicates duration (ms) during which a PDCP SDUacquired from the upper layer is valid. When the discardTimer is expiredor the successful delivery of a PDCP SDU is confirmed by PDCP statusreport, the UE discards the PDCP SDU.

The rlc-AM field is a field that is necessary to setup a PDCP entity fora Radio bearer configured with the RLC Acknowledge Mode (AM). The rlc-AMincludes “statusReportRequired” indicating whether the UE shouldtransmit a PDCP status report in response to a PDCP entityre-establishment and a PDCP data recovery.

The rlc-UM field is a field that is necessary to setup a PDCP entity fora Radio bearer configured with the RLC Unacknowledged Mode (UM). Therlc-UM includes pdcp-SN-Size (i.e., 7 bits, 12 bits, 15 bits, or 18bits).

The headerCompression field includes robust header compression (ROHC)information to be used for Header Compression performed in the PDCPlayer. The ROHC information further includes a maximum ContextIdentifier (maxCID) and profiles. The profiles define a specificcombination of the protocols of the network layer, transport layer, andupper layer thereof.

The rn-IntegrityProtection field indicates whether integrity protectionor verification shall be applied for all subsequent packets received andsent by a Relay node.

The ul-DataSplitDRB-ViaSCG field indicates whether the UE shall transmitPDCP PDUs via SCG.

The t-Reordering field indicates the value (ms) of a Reordering Timer.

On the other hand, the NR PDCP-config may include at least one of theinformation elements included in the existing LTE PDCP-config. Forexample, as described above, the number of the possible values of thepdcp-SN-Size within the NR PDCP-config may be smaller (i.e., 12 bits or18 bits) than that of the possible values of the pdcp-SN-Size in the NRPDCP-config. In addition, or alternatively, the NR PDCP-config mayinclude an additional information element that is not included in theexisting LTE PDCP-config. The additional information element may include“ul-DataSplitDRB-ViaUnifiedSplitBearer” (oru“l-DataSplitDRB-ViaMCGSplitBearer”, or“ul-DataSplitDRB-ViaSCGSplitBearer”) indicating whether the UE shalltransmit PDCP PDUs via a unified split bearer (or an MCG split bearer,or an SCG Split bearer). The additional information element may includeinformation regarding acquisition of SDAP PDUs from an SDAP sublayer, ordelivery of PDCP SDUs to an SDAP sublayer.

As described above, the Unified PDCP config may be the same as the NRPDCP-config or the LTE PDCP-config, may be a subset of the NRPDCP-config or the LTE PDCP-config, or may be a common subset (commonpart) between the NR PDCP-config and the LTE PDCP-config.

The NR PDCP-config may be included in the LTE PDCP-config as a subsetthereof and transmitted from the MN 1 to the UE 3, or it may betransmitted from the MN 1 to the UE 3 in addition to the LTEPDCP-config.

The Unified PDCP config may be included in the LTE PDCP-config or the NRPDCP-config as a subset thereof and transmitted from the MN 1 to the UE3, or it may be transmitted from the MN 1 to the UE 3 in addition to theLTE PDCP-config and the NR PDCP-config.

When the Unified PDCP config is included in each of the LTE PDCP-configand the NR PDCP-config as a subset thereof, the UE 3 may recognize thatthe Unified PDCP config is activated under a condition that the twoUnified PDCP configs respectively included in the LTE PDCP-config andthe NR PDCP-config at least partially coincide with one another.

The following provides configuration examples of the MN 1, the SN 2, theUE 3, and the CP node 5 according to the above-described embodiments.FIG. 14 is a block diagram showing a configuration example of the MN 1according to the above-described embodiments. The configuration of theSN 2 may be similar to that shown in FIG. 14. Referring to FIG. 14, theMN 1 includes a Radio Frequency transceiver 1401, a network interface1403, a processor 1404, and a memory 1405. The RF transceiver 1401performs analog RF signal processing to communicate with UEs includingthe UE 3. The RF transceiver 1401 may include a plurality oftransceivers. The RF transceiver 1401 is coupled to an antenna array1402 and the processor 1404. The RF transceiver 1401 receives modulatedsymbol data from the processor 1404, generates a transmission RF signal,and supplies the transmission RF signal to the antenna array 1402.Further, the RF transceiver 1401 generates a baseband reception signalbased on a reception RF signal received by the antenna array 1402 andsupplies the baseband reception signal to the processor 1404.

The network interface 1403 is used to communicate with network nodes(e.g., the SN 2, the CP node 5, and the UP node 6). The networkinterface 1403 may include, for example, a network interface card (NIC)conforming to the IEEE 802.3 series.

The processor 1404 performs digital baseband signal processing (i.e.,data-plane processing) and control-plane processing for radiocommunication. The processor 1404 may include a plurality of processors.The processor 1404 may include, for example, a modem processor (e.g., aDigital Signal Processor (DSP)) that performs digital baseband signalprocessing and a protocol stack processor (e.g., a Central ProcessingUnit (CPU) or a Micro Processing Unit (MPU)) that performs thecontrol-plane processing.

The memory 1405 is composed of a combination of a volatile memory and anon-volatile memory. The volatile memory is, for example, a StaticRandom Access Memory (SRAM), a Dynamic RAM (DRAM), or a combinationthereof. The non-volatile memory is, for example, a mask Read OnlyMemory (MROM), an Electrically Erasable Programmable ROM (EEPROM), aflash memory, a hard disc drive, or any combination thereof. The memory1405 may include a storage located apart from the processor 1404. Inthis case, the processor 1404 may access the memory 1405 via the networkinterface 1403 or an I/O interface (not shown).

The memory 1405 may store one or more software modules (computerprograms) 1406 including instructions and data to perform processing bythe MN 1 described in the above-described embodiments. In someimplementations, the processor 1404 may be configured to load thesoftware modules 1406 from the memory 1405 and execute the loadedsoftware modules, thereby performing processing of the MN 1 described inthe above-described embodiments.

FIG. 15 is a block diagram showing a configuration example of the UE 3.A Radio Frequency (RF) transceiver 1501 performs analog RF signalprocessing to communicate with the MN 1 and the SN 2. The RF transceiver1501 may include a plurality of transceivers. The analog RF signalprocessing performed by the RF transceiver 1501 includes frequencyup-conversion, frequency down-conversion, and amplification. The RFtransceiver 1501 is coupled to an antenna array 1502 and a basebandprocessor 1503. The RF transceiver 1501 receives modulated symbol data(or OFDM symbol data) from the baseband processor 1503, generates atransmission RF signal, and supplies the transmission RF signal to theantenna array 1502. Further, the RF transceiver 1501 generates abaseband reception signal based on a reception RF signal received by theantenna array 1502 and supplies the baseband reception signal to thebaseband processor 1503.

The baseband processor 1503 performs digital baseband signal processing(i.e., data-plane processing) and control-plane processing for radiocommunication. The digital baseband signal processing includes (a) datacompression/decompression, (b) data segmentation/concatenation, (c)composition/decomposition of a transmission format (i.e., transmissionframe), (d) channel coding/decoding, (e) modulation (i.e., symbolmapping)/demodulation, and (f) generation of OFDM symbol data (i.e.,baseband OFDM signal) by Inverse Fast Fourier Transform (IFFT).Meanwhile, the control-plane processing includes communicationmanagement of layer 1 (e.g., transmission power control), layer 2 (e.g.,radio resource management and hybrid automatic repeat request (HARQ)processing), and layer 3 (e.g., signaling regarding attach, mobility,and call management).

The digital baseband signal processing by the baseband processor 1503may include, for example, signal processing of a Packet Data ConvergenceProtocol (PDCP) layer, a Radio Link Control (RLC) layer, a MAC layer,and a PHY layer. Further, the control-plane processing performed by thebaseband processor 1503 may include processing of a Non-Access Stratum(NAS) protocol, an RRC protocol, and MAC CEs.

The baseband processor 1503 may include a modem processor (e.g., DSP)that performs the digital baseband signal processing and a protocolstack processor (e.g., a CPU or an MPU) that performs the control-planeprocessing. In this case, the protocol stack processor, which performsthe control-plane processing, may be integrated with an applicationprocessor 1504 described in the following.

The application processor 1504 is also referred to as a CPU, an MPU, amicroprocessor, or a processor core. The application processor 1504 mayinclude a plurality of processors (processor cores). The applicationprocessor 1504 loads a system software program (Operating System (OS))and various application programs (e.g., a call application, a WEBbrowser, a mailer, a camera operation application, and a music playerapplication) from a memory 1506 or from another memory (not shown) andexecutes these programs, thereby providing various functions of the UE3.

In some implementations, as represented by a dashed line (1505) in FIG.15, the baseband processor 1503 and the application processor 1504 maybe integrated on a single chip. In other words, the baseband processor1503 and the application processor 1504 may be implemented in a singleSystem on Chip (SoC) device 1505. An SoC device may be referred to as aLarge Scale Integration (LSI) or a chipset.

The memory 1506 is a volatile memory, a non-volatile memory, or acombination thereof. The memory 1506 may include a plurality of memorydevices that are physically independent from each other. The volatilememory is, for example, an SRAM, a DRAM, or a combination thereof. Thenon-volatile memory is, for example, an MROM, an EEPROM, a flash memory,a hard disc drive, or any combination thereof. The memory 1506 mayinclude, for example, an external memory device that can be accessedfrom the baseband processor 1503, the application processor 1504, andthe SoC 1505. The memory 1506 may include an internal memory device thatis integrated in the baseband processor 1503, the application processor1504, or the SoC 1505. Further, the memory 1506 may include a memory ina Universal Integrated Circuit Card (UICC).

The memory 1506 may store one or more software modules (computerprograms) 1507 including instructions and data to perform the processingby the UE 3 described in the above-described embodiments. In someimplementations, the baseband processor 1503 or the applicationprocessor 1504 may load these software modules 1507 from the memory 1506and execute the loaded software modules, thereby performing theprocessing of the UE 3 described in the above-described embodiments withreference to the drawings.

FIG. 16 is a block diagram showing a configuration example of the CPnode 5 according to the above-described embodiments. Referring to FIG.16, the CP node 5 includes a network interface 1601, a processor 1602,and a memory 1603. The network interface 1601 is used to communicatewith network nodes (e.g., RAN nodes and other core network nodes). Thenetwork interface 1601 may include, for example, a network interfacecard (NIC) conforming to the IEEE 802.3 series.

The processor 1602 may be, for example, a microprocessor, an MPU, or aCPU. The processor 1602 may include a plurality of processors.

The memory 1603 is composed of a combination of a volatile memory and anonvolatile memory. The volatile memory is, for example, an SRAM, aDRAM, or a combination thereof. The non-volatile memory is, for example,an MROM, a PROM, a flash memory, a hard disc drive, or any combinationthereof. The memory 1603 may include a storage located apart from theprocessor 1602. In this case, the processor 1602 may access the memory1603 via the network interface 1601 or an I/O interface (not shown).

The memory 1603 may store one or more software modules (computerprograms) 1604 including instructions and data to perform the processingof the CP node 5 described in the above-described embodiments. In someimplementations, the processor 1602 may be configured to load the one ormore software modules 1604 from the memory 1603 and execute the loadedsoftware modules, thereby performing the processing of the CP node 5described in the above-described embodiments.

As described above with reference to FIGS. 14, 15, and 16, each of theprocessors included in the MN 1, the SN 2, the UE 3, and the CP node 5according to the above-described embodiments executes one or moreprograms including instructions to cause a computer to perform analgorithm described with reference to the drawings. The program(s) canbe stored and provided to a computer using any type of non-transitorycomputer readable media. Non-transitory computer readable media includeany type of tangible storage media. Examples of non-transitory computerreadable media include magnetic storage media (such as flexible disks,magnetic tapes, hard disk drives, etc.), optical magnetic storage media(e.g., magneto-optical disks), Compact Disc Read Only Memory (CD-ROM),CD-R, CD-R/W, and semiconductor memories (such as mask ROM, ProgrammableROM (PROM), Erasable PROM (EPROM), flash ROM, Random Access Memory(RAM), etc.). The program(s) may be provided to a computer using anytype of transitory computer readable media. Examples of transitorycomputer readable media include electric signals, optical signals, andelectromagnetic waves. Transitory computer readable media can providethe program to a computer via a wired communication line (e.g., electricwires, and optical fibers) or a wireless communication line.

Other Embodiments

The above-described embodiments have mainly described as to the examplesof the EN-DC. The configurations and operations of the apparatusesdescribed in these embodiments may be used for the NE-DC and theNG-EN-DC.

The above-described embodiments have provided the examples in which theinformation elements (e.g., SCG-ConfigInfo, SCG-Config) and messagestransmitted between the MN 1 and the SN 2 have the names and theconfigurations assuming the LTE DC. However, the names and theconfigurations of the information elements and message for the MR-DC maynot be the same as those of the LTE DC. For example, at least some ofthe information elements included in the SCG-ConfigInfo and theSCG-Config may be defined to be information elements of the X2 interface(or the Xn interface) between the MN 1 and the SN 2.

The above-described embodiments have been mainly described as to DRBsand MCG SRBs. However, the unified PDCP described in the above-describedembodiments may be used for other radio bearers including an MCG SplitSRB and an SCG SRB. Further, the configuration (or establishment) of theunified PDCP may be implicitly indicated by the configuration of thesplit bearer (e.g., DRB configuration with drbType “split”).Alternatively, the flag “unified” to explicitly indicate configuration(or establishment) of the unified PDCP may be added to the PDCPconfiguration (PDCP Config).

The above-described embodiments may be applied to a mobility scenario.When the Mobility procedure is performed, for example, the MN 1, the SN2, and the UE 3 may change (or fall back) from the unified PDCP to theexisting PDCP (e.g. LTE PDCP). The mobility here may include one or bothof an intra-SN change (e.g. PSCell change) and an inter-SN change(change of SN), as well as a handover. The target to be controlled heremay be all bearers that are somewhat affected by the mobility. Forexample, when a handover is performed, all bearers that use the unifiedPDCP may be treated as a target of the PDCP change control. When anIntra-/inter-SN change is performed, SCG split bearers may be treated asa target of the PDCP change control. Alternatively, when the UE 3 thatuses the unified PDCP in the source cell performs a handover to thetarget cell, the UE 3 may continuously use the unified PDCP, except forwhen the target cell does not support a split bearer (i.e., unifiedbearer or unified PDCP). When the target cell does not support a splitbearer, the UE 3 may change (fall back) from the unified PDCP to theexisting PDCP (e.g. LTE PDCP).

In the above-described embodiments, when the unified PDCP includesfunctions and processing different from those of the existing PDCP(i.e., MN PDCP or SN PDCP), another (sub) layer may operate in view ofthis difference, or the unified PDCP may operate in view of thisdifference. For example, while the amount of data waiting fortransmission (amount of buffer) between the LTE PDCP and MAC has beendefined to be “data available for transmission”, it has been discussedto specify the amount of data waiting for transmission between the NRPDCP and MAC as “data volume”. For example, when the NR PDCP is used asthe unified PDCP, one or both of the LTE MAC and the NR PDCP (i.e.,unified PDCP) in the MeNB and the UE MCG of the EN-DC may operate inview of this difference.

The MN 1 and the SN 2 described in the above embodiments may beimplemented based on a Cloud Radio Access Network (C-RAN) concept. TheC-RAN is also referred to as a Centralized RAN. In this case, processesand operations performed by each of the MN 1 and the SN 2 described inthe above-described embodiments may be provided by a Digital Unit (DU)included in the C-RAN architecture, or by a combination of a DU and aRadio Unit (RU). The DU is also referred to as a Baseband Unit (BBU) ora Central Unit (CU). The RU is also referred to as a Remote Radio Head(RRH), a Remote Radio Equipment (RRE), a Distributed Unit (DU), or aTransmission and Reception Point (TRP). That is, processes andoperations performed by each of the MN 1 and the SN 2 described in theabove-described embodiments may be provided by one or more radiostations (or RAN nodes).

Further, the above-described embodiments are merely examples ofapplications of the technical ideas obtained by the inventor. Thesetechnical ideas are not limited to the above-described embodiments andvarious modifications may be made thereto.

For example, the whole or part of the above-described embodiments can bedescribed as, but not limited to, the following supplementary notes.

(Supplementary Note 1)

A master radio access network (RAN) node associated with a master radioaccess technology (RAT), the master RAN node comprising:

a memory; and

at least one processor coupled to the memory and configured to:

communicate with a secondary RAN node associated with a secondary RATand provide a radio terminal with dual connectivity that uses the masterRAT and the secondary RAT;

if the radio terminal does not support a split bearer, use, for a mastercell group bearer for the radio terminal, a PDCP entity that providesfirst Packet Data Convergence Protocol (PDCP) functionalitiescorresponding to the master RAT; and if the radio terminal supports thesplit bearer, use, for the master cell group bearer for the radioterminal, a PDCP entity that provides unified PDCP functionalities,regardless of whether the dual connectivity is started for the radioterminal, wherein the unified PDCP functionalities are used for both amaster cell group split bearer and a secondary cell group split bearer,the master cell group split bearer is a user plane bearer whose radioprotocols are split at the master RAN node and belong to both a mastercell group provided by the master RAN node and a secondary cell groupprovided by the secondary RAN node, the secondary cell group splitbearer is a user plane bearer whose radio protocols are split at thesecondary RAN node and belong to both the secondary cell group and themaster cell group, and the master cell group bearer is a user planebearer whose radio protocols are only located in the master cell group.

(Supplementary Note 2)

The master RAN node according to Supplementary Note 1, wherein theunified PDCP functionalities are common functionalities that both thefirst PDCP functionalities corresponding to the master RAT and secondPDCP functionalities corresponding to the secondary RAT have.

(Supplementary Note 3)

The master RAN node according to Supplementary Note 1, wherein theunified PDCP functionalities are a common subset between the first PDCPfunctionalities corresponding to the master RAT and second PDCPfunctionalities corresponding to the secondary RAT.

(Supplementary Note 4)

The master RAN node according to Supplementary Note 1, wherein

second PDCP functionalities corresponding to the secondary RAT are asubset of the first PDCP functionalities corresponding to the masterRAT, and

the unified PDCP functionalities are the same as, or a subset of, thesecond PDCP functionalities corresponding to the secondary RAT.

(Supplementary Note 5)

The master RAN node according to any one of Supplementary Notes 1 to 4,wherein the at least one processor is configured to receive from theradio terminal, during a Radio Resource Control (RRC) connectionestablishment procedure, terminal capability information indicatingwhether the radio terminal supports a split bearer.

(Supplementary Note 6)

The master RAN node according to Supplementary Note 5, wherein the atleast one processor is configured to:

-   -   receive from the radio terminal an RRC connection request        message that contains the terminal capability information; and    -   when the terminal capability information indicates that the        radio terminal supports a split bearer, transmit to the radio        terminal an RRC connection setup message that contains        information indicating use of the unified PDCP functionalities        or contains a PDCP configuration corresponding to the unified        PDCP functionalities.

(Supplementary Note 7)

The master RAN node according to Supplementary Note 6, wherein the atleast one processor is configured to perform access stratum securityactivation with the radio terminal via a signalling radio bearer towhich the PDCP configuration corresponding to the unified PDCPfunctionalities is applied.

(Supplementary Note 8)

The master RAN node according to any one of Supplementary Notes 5 to 7,wherein the terminal capability information is defined to be an RRCinformation element.

(Supplementary Note 9)

The master RAN node according to any one of Supplementary Notes 5 to 7,wherein the terminal capability information is defined to be a MediumAccess Control (MAC) Control Element (CE).

(Supplementary Note 10)

The master RAN node according to any one of Supplementary Notes 5 to 7,wherein the terminal capability information is defined to be a LogicalChannel ID (LCID) of a Common Control Channel (CCCH).

(Supplementary Note 11)

A radio terminal comprising:

-   -   at least one wireless transceiver configured to communicate with        both a master radio access network (RAN) node associated with a        master radio access technology (RAT) and a secondary RAN node        associated with a secondary RAT; and    -   at least one processor configured to:        -   perform, via the at least one wireless transceiver, dual            connectivity that uses the master RAT and the secondary RAT;        -   if the radio terminal does not support a split bearer, use,            for a master cell group bearer for the radio terminal, a            PDCP entity that provides first Packet Data Convergence            Protocol (PDCP) functionalities corresponding to the master            RAT; and        -   if the radio terminal supports the split bearer, use, for            the master cell group bearer for the radio terminal, a PDCP            entity that provides unified PDCP functionalities,            regardless of whether the dual connectivity is started for            the radio terminal, wherein    -   the unified PDCP functionalities are used for both a master cell        group split bearer and a secondary cell group split bearer,    -   the master cell group split bearer is a user plane bearer whose        radio protocols are split at the master RAN node and belong to        both a master cell group provided by the master RAN node and a        secondary cell group provided by the secondary RAN node,    -   the secondary cell group split bearer is a user plane bearer        whose radio protocols are split at the secondary RAN node and        belong to both the secondary cell group and the master cell        group, and    -   the master cell group bearer is a user plane bearer whose radio        protocols are only located in the master cell group.

(Supplementary Note 12)

The radio terminal according to Supplementary Note 11, wherein the atleast one processor is configured to transmit to the master RAN node,during a Radio Resource Control (RRC) connection establishmentprocedure, terminal capability information indicating that the radioterminal supports a split bearer.

(Supplementary Note 13)

The radio terminal according to Supplementary Note 12, wherein the atleast one processor is configured to:

-   -   transmit to the master RAN node an RRC connection request        message that contains the terminal capability information; and    -   receive from the master RAN node an RRC connection setup message        that contains information indicating use of the unified PDCP        functionalities or contains a PDCP configuration corresponding        to the unified PDCP functionalities.

(Supplementary Note 14)

The radio terminal according to Supplementary Note 13, wherein the atleast one processor is configured to perform access stratum securityactivation with the master RAN node via a signalling radio bearer towhich the PDCP configuration corresponding to the unified PDCPfunctionalities is applied.

(Supplementary Note 15)

A method for a master radio access network (RAN) node associated with amaster radio access technology (RAT), the method comprising:

-   -   communicating with a secondary RAN node associated with a        secondary RAT and providing a radio terminal with dual        connectivity that uses the master RAT and the secondary RAT;    -   if the radio terminal does not support a split bearer, using,        for a master cell group bearer for the radio terminal, a PDCP        entity that provides first Packet Data Convergence Protocol        (PDCP) functionalities corresponding to the master RAT; and    -   if the radio terminal supports the split bearer, using, for the        master cell group bearer for the radio terminal, a PDCP entity        that provides unified PDCP functionalities, regardless of        whether the dual connectivity is started for the radio terminal,        wherein    -   the unified PDCP functionalities are used for both a master cell        group split bearer and a secondary cell group split bearer,    -   the master cell group split bearer is a user plane bearer whose        radio protocols are split at the master RAN node and belong to        both a master cell group provided by the master RAN node and a        secondary cell group provided by the secondary RAN node,    -   the secondary cell group split bearer is a user plane bearer        whose radio protocols are split at the secondary RAN node and        belong to both the secondary cell group and the master cell        group, and    -   the master cell group bearer is a user plane bearer whose radio        protocols are only located in the master cell group.

(Supplementary Note 16)

A method for a radio terminal, the method comprising:

-   -   performing dual connectivity that uses a master radio access        technology (RAT) and a secondary RAT via a wireless transceiver        configured to communicate with both a master radio access        network (RAN) node associated with the master RAT and a        secondary RAN node associated with the secondary RAT;    -   if the radio terminal does not support a split bearer, using,        for a master cell group bearer for the radio terminal, a PDCP        entity that provides first Packet Data Convergence Protocol        (PDCP) functionalities corresponding to the master RAT; and    -   if the radio terminal supports the split bearer, using, for the        master cell group bearer for the radio terminal, a PDCP entity        that provides unified PDCP functionalities, regardless of        whether the dual connectivity is started for the radio terminal,        wherein    -   the unified PDCP functionalities are used for both a master cell        group split bearer and a secondary cell group split bearer,    -   the master cell group split bearer is a user plane bearer whose        radio protocols are split at the master RAN node and belong to        both a master cell group provided by the master RAN node and a        secondary cell group provided by the secondary RAN node,    -   the secondary cell group split bearer is a user plane bearer        whose radio protocols are split at the secondary RAN node and        belong to both the secondary cell group and the master cell        group, and    -   the master cell group bearer is a user plane bearer whose radio        protocols are only located in the master cell group.

(Supplementary Note 17)

A program for causing a computer to perform a method for a master radioaccess network (RAN) node associated with a master radio accesstechnology (RAT), wherein the method comprises:

-   -   communicating with a secondary RAN node associated with a        secondary RAT and providing a radio terminal with dual        connectivity that uses the master RAT and the secondary RAT;    -   if the radio terminal does not support a split bearer, using,        for a master cell group bearer for the radio terminal, a PDCP        entity that provides first Packet Data Convergence Protocol        (PDCP) functionalities corresponding to the master RAT; and    -   if the radio terminal supports the split bearer, using, for the        master cell group bearer for the radio terminal, a PDCP entity        that provides unified PDCP functionalities, regardless of        whether the dual connectivity is started for the radio terminal,        wherein    -   the unified PDCP functionalities are used for both a master cell        group split bearer and a secondary cell group split bearer,    -   the master cell group split bearer is a user plane bearer whose        radio protocols are split at the master RAN node and belong to        both a master cell group provided by the master RAN node and a        secondary cell group provided by the secondary RAN node,    -   the secondary cell group split bearer is a user plane bearer        whose radio protocols are split at the secondary RAN node and        belong to both the secondary cell group and the master cell        group, and    -   the master cell group bearer is a user plane bearer whose radio        protocols are only located in the master cell group.

(Supplementary Note 18)

A program for causing a computer to perform a method for a radioterminal, wherein the method comprises:

-   -   performing dual connectivity that uses a master radio access        technology (RAT) and a secondary RAT via a wireless transceiver        configured to communicate with both a master radio access        network (RAN) node associated with the master RAT and a        secondary RAN node associated with the secondary RAT;    -   if the radio terminal does not support a split bearer, using,        for a master cell group bearer for the radio terminal, a PDCP        entity that provides first Packet Data Convergence Protocol        (PDCP) functionalities corresponding to the master RAT; and    -   if the radio terminal supports the split bearer, using, for the        master cell group bearer for the radio terminal, a PDCP entity        that provides unified PDCP functionalities, regardless of        whether the dual connectivity is started for the radio terminal,        wherein    -   the unified PDCP functionalities are used for both a master cell        group split bearer and a secondary cell group split bearer,    -   the master cell group split bearer is a user plane bearer whose        radio protocols are split at the master RAN node and belong to        both a master cell group provided by the master RAN node and a        secondary cell group provided by the secondary RAN node,    -   the secondary cell group split bearer is a user plane bearer        whose radio protocols are split at the secondary RAN node and        belong to both the secondary cell group and the master cell        group, and    -   the master cell group bearer is a user plane bearer whose radio        protocols are only located in the master cell group.

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2017-118095, filed on Jun. 15, 2017, thedisclosure of which is incorporated herein in its entirety by reference.

REFERENCE SIGNS LIST

-   1 MASTER NODE (MN)-   2 SECONDARY NODE (SN)-   3 USER EQUIPMENT (UE)-   4 CORE NETWORK-   5 CONTROL PLANE (CP) NODE-   6 USER PLANE (UP) NODE-   1401 RF TRANSCEIVER-   1404 PROCESSOR-   1405 MEMORY-   1501 RF TRANSCEIVER-   1503 BASEBAND PROCESSOR-   1504 APPLICATION PROCESSOR-   1506 MEMORY-   1602 PROCESSOR-   1603 MEMORY

1. A master radio access network (RAN) node associated with a masterradio access technology (RAT), the master RAN node comprising: a memory;and at least one processor coupled to the memory and configured to:communicate with a secondary RAN node associated with a secondary RATand provide a radio terminal with dual connectivity that uses the masterRAT and the secondary RAT; and in response to receiving, from the radioterminal or a core network, terminal capability information indicatingthat the radio terminal supports a split bearer, use a Packet DataConvergence Protocol (PDCP) entity for a master cell group split bearerfor the radio terminal, the PDCP entity providing unified PDCPfunctionalities, wherein the unified PDCP functionalities are used forboth the master cell group split bearer and a secondary cell group splitbearer, the master cell group split bearer is a user plane bearer whoseradio protocols are split at the master RAN node and belong to both amaster cell group provided by the master RAN node and a secondary cellgroup provided by the secondary RAN node, and the secondary cell groupsplit bearer is a user plane bearer whose radio protocols are split atthe secondary RAN node and belong to both the secondary cell group andthe master cell group.
 2. The master RAN node according to claim 1,wherein the unified PDCP functionalities are common functionalities thatboth first PDCP functionalities corresponding to the master RAT andsecond PDCP functionalities corresponding to the secondary RAT have. 3.The master RAN node according to claim 1, wherein the unified PDCPfunctionalities are a common subset between first PDCP functionalitiescorresponding to the master RAT and second PDCP functionalitiescorresponding to the secondary RAT.
 4. The master RAN node according toclaim 1, wherein second PDCP functionalities corresponding to thesecondary RAT are a subset of first PDCP functionalities correspondingto the master RAT, and the unified PDCP functionalities are the same as,or a subset of, the second PDCP functionalities corresponding to thesecondary RAT.
 5. The master RAN node according to claim 1, wherein theat least one processor is configured to prepare the PDCP entity thatprovides the unified PDCP functionalities by establishing,re-establishing, or reconfiguring a PDCP entity with a configurationcommon to both first PDCP functionalities corresponding to the masterRAT and second PDCP functionalities corresponding to the secondary RAT.6. The master RAN node according to claim 1, wherein the at least oneprocessor is configured to establish, re-establish, or reconfigure aPDCP entity for an already established master cell group bearer, or toswitch a mode of the PDCP entity for the master cell group bearer, insuch a way that it provides the unified PDCP functionalities, therebyestablishing the PDCP entity that provides the unified PDCPfunctionalities for the master cell group split bearer, for thesecondary cell group split bearer, or for a secondary cell group bearer,wherein the master cell group bearer is a user plane bearer whose radioprotocols are only located in the master cell group, and the secondarycell group bearer is a user plane bearer whose radio protocols are onlylocated in the secondary cell group.
 7. The master RAN node according toclaim 1, wherein the at least one processor is configured to, if theradio terminal supports a split bearer, use the PDCP entity thatprovides the unified PDCP functionalities also for a master cell groupbearer for the radio terminal, regardless of whether the dualconnectivity is started for the radio terminal, wherein the master cellgroup bearer is a user plane bearer whose radio protocols are onlylocated in the master cell group.
 8. The master RAN node according toclaim 7, wherein the at least one processor is configured to receive theterminal capability information from the radio terminal during a RadioResource Control (RRC) connection establishment procedure.
 9. The masterRAN node according to claim 8, wherein the at least one processor isconfigured to: receive from the radio terminal an RRC connection requestmessage that contains the terminal capability information; and when theterminal capability information indicates that the radio terminalsupports a split bearer, transmit to the radio terminal an RRCconnection setup message that contains information indicating use of theunified PDCP functionalities or contains a PDCP configurationcorresponding to the unified PDCP functionalities.
 10. The master RANnode according to claim 9, wherein the at least one processor isconfigured to perform access stratum security activation with the radioterminal via a signalling radio bearer to which the PDCP configurationcorresponding to the unified PDCP functionalities is applied.
 11. Themaster RAN node according to claim 8, wherein the terminal capabilityinformation is defined to be an RRC information element. 12-13.(canceled)
 14. A secondary radio access network (RAN) node configured tosupport a secondary radio access technology (RAT), the secondary RANnode comprising: a memory; and at least one processor coupled to thememory and configured to: communicate with a master RAN node thatsupports a master RAT and provide a radio terminal with dualconnectivity that uses the master RAT and the secondary RAT; and if themaster RAN node receives, from the radio terminal or a core network,terminal capability information indicating that the radio terminalsupports a split bearer, use a Packet Data Convergence Protocol (PDCP)entity for a secondary cell group split bearer for the radio terminal,the PDCP entity providing unified PDCP functionalities, wherein theunified PDCP functionalities are used for both the secondary cell groupsplit bearer and a master cell group split bearer, the master cell groupsplit bearer is a user plane bearer whose radio protocols are split atthe master RAN node and belong to both a master cell group provided bythe master RAN node and a secondary cell group provided by the secondaryRAN node, and the secondary cell group split bearer is a user planebearer whose radio protocols are split at the secondary RAN node andbelong to both the secondary cell group and the master cell group. 15.The secondary RAN node according to claim 14, wherein the at least oneprocessor is configured to establish, re-establish, or reconfigure aPDCP entity for an already established secondary cell group bearer, orswitch a mode of the PDCP entity for the secondary cell group bearer, insuch a way that it provides the unified PDCP functionalities, therebyestablishing the PDCP entity that provides the unified PDCPfunctionalities for the secondary cell group split bearer, wherein thesecondary cell group bearer is a user plane bearer whose radio protocolsare only located in the secondary cell group.
 16. A radio terminalcomprising: at least one wireless transceiver configured to communicatewith both a master radio access network (RAN) node associated with amaster radio access technology (RAT) and a secondary RAN node associatedwith a secondary RAT; and at least one processor configured to: perform,via the at least one wireless transceiver, dual connectivity that usesthe master RAT and the secondary RAT; if the radio terminal supports asplit bearer, transmit, to the master RAN node, terminal capabilityinformation indicating that the radio terminal supports a split bearer;and if the radio terminal supports a split bearer, use a Packet DataConvergence Protocol (PDCP) entity for a master cell group split bearerfor the radio terminal, the PDCP entity providing unified PDCPfunctionalities, wherein the unified PDCP functionalities are used forboth the master cell group split bearer and a secondary cell group splitbearer, the master cell group split bearer is a user plane bearer whoseradio protocols are split at the master RAN node and belong to both amaster cell group provided by the master RAN node and a secondary cellgroup provided by the secondary RAN node, and the secondary cell groupsplit bearer is a user plane bearer whose radio protocols are split atthe secondary RAN node and belong to both the secondary cell group andthe master cell group.
 17. The radio terminal according to claim 16,wherein the at least one processor is configured to establish,re-establish, or reconfigure a PDCP entity for an already establishedmaster cell group bearer, or to switch a mode of the PDCP entity for themaster cell group bearer, in such a way that it provides the unifiedPDCP functionalities, thereby establishing the PDCP entity that providesthe unified PDCP functionalities for the master cell group split bearer,for the secondary cell group split bearer, or for a secondary cell groupbearer, wherein the master cell group bearer is a user plane bearerwhose radio protocols are only located in the master cell group, and thesecondary cell group bearer is a user plane bearer whose radio protocolsare only located in the secondary cell group.
 18. The radio terminalaccording to claim 16, wherein the at least one processor is configuredto, if the radio terminal supports a split bearer, use the PDCP entitythat provides the unified PDCP functionalities also for a master cellgroup bearer for the radio terminal, regardless of whether the dualconnectivity is started for the radio terminal, wherein the master cellgroup bearer is a user plane bearer whose radio protocols are onlylocated in the master cell group.
 19. The radio terminal according toclaim 18, wherein the at least one processor is configured to transmitthe terminal capability information to the master RAN node during aRadio Resource Control (RRC) connection establishment procedure.
 20. Theradio terminal according to claim 19, wherein the at least one processoris configured to: transmit to the master RAN node an RRC connectionrequest message that contains the terminal capability information; andreceive from the master RAN node an RRC connection setup message thatcontains information indicating use of the unified PDCP functionalitiesor contains a PDCP configuration corresponding to the unified PDCPfunctionalities.
 21. The radio terminal according to claim 20, whereinthe at least one processor is configured to perform access stratumsecurity activation with the master RAN node via a signalling radiobearer to which the PDCP configuration corresponding to the unified PDCPfunctionalities is applied.
 22. A method for a master radio accessnetwork (RAN) node associated with a master radio access technology(RAT), the method comprising: communicating with a secondary RAN nodeassociated with a secondary RAT and providing a radio terminal with dualconnectivity that uses the master RAT and the secondary RAT; and inresponse to receiving, from the radio terminal or a core network,terminal capability information indicating that the radio terminalsupports a split bearer, using a Packet Data Convergence Protocol (PDCP)entity for a master cell group split bearer for the radio terminal, thePDCP entity providing unified PDCP functionalities, wherein the unifiedPDCP functionalities are used for both the master cell group splitbearer and a secondary cell group split bearer, the master cell groupsplit bearer is a user plane bearer whose radio protocols are split atthe master RAN node and belong to both a master cell group provided bythe master RAN node and a secondary cell group provided by the secondaryRAN node, and the secondary cell group split bearer is a user planebearer whose radio protocols are split at the secondary RAN node andbelong to both the secondary cell group and the master cell group.23-45. (canceled)